P. 147 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 14601-14700   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	xmetge0 14601	The distance function of a metric space is nonnegative. (Contributed by Mario Carneiro, 20-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ A e. X /\ B e. X ) -> 0 <_ ( A D B ) )
Theorem	metge0 14602	The distance function of a metric space is nonnegative. (Contributed by NM, 27-Aug-2006.) (Revised by Mario Carneiro, 14-Aug-2015.)
|- ( ( D e. ( Met ` X ) /\ A e. X /\ B e. X ) -> 0 <_ ( A D B ) )
Theorem	xmetlecl 14603	Real closure of an extended metric value that is upper bounded by a real. (Contributed by Mario Carneiro, 20-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ ( A e. X /\ B e. X ) /\ ( C e. RR /\ ( A D B ) <_ C ) ) -> ( A D B ) e. RR )
Theorem	xmetsym 14604	The distance function of an extended metric space is symmetric. (Contributed by Mario Carneiro, 20-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ A e. X /\ B e. X ) -> ( A D B ) = ( B D A ) )
Theorem	xmetpsmet 14605	An extended metric is a pseudometric. (Contributed by Thierry Arnoux, 7-Feb-2018.)
|- ( D e. ( *Met ` X ) -> D e. (PsMet ` X ) )
Theorem	xmettpos 14606	The distance function of an extended metric space is symmetric. (Contributed by Mario Carneiro, 20-Aug-2015.)
|- ( D e. ( *Met ` X ) -> tpos D = D )
Theorem	metsym 14607	The distance function of a metric space is symmetric. Definition 14-1.1(c) of [Gleason] p. 223. (Contributed by NM, 27-Aug-2006.) (Revised by Mario Carneiro, 20-Aug-2015.)
|- ( ( D e. ( Met ` X ) /\ A e. X /\ B e. X ) -> ( A D B ) = ( B D A ) )
Theorem	xmettri 14608	Triangle inequality for the distance function of a metric space. Definition 14-1.1(d) of [Gleason] p. 223. (Contributed by Mario Carneiro, 20-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ ( A e. X /\ B e. X /\ C e. X ) ) -> ( A D B ) <_ ( ( A D C ) +e ( C D B ) ) )
Theorem	mettri 14609	Triangle inequality for the distance function of a metric space. Definition 14-1.1(d) of [Gleason] p. 223. (Contributed by NM, 27-Aug-2006.)
|- ( ( D e. ( Met ` X ) /\ ( A e. X /\ B e. X /\ C e. X ) ) -> ( A D B ) <_ ( ( A D C ) + ( C D B ) ) )
Theorem	xmettri3 14610	Triangle inequality for the distance function of an extended metric. (Contributed by Mario Carneiro, 20-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ ( A e. X /\ B e. X /\ C e. X ) ) -> ( A D B ) <_ ( ( A D C ) +e ( B D C ) ) )
Theorem	mettri3 14611	Triangle inequality for the distance function of a metric space. (Contributed by NM, 13-Mar-2007.)
|- ( ( D e. ( Met ` X ) /\ ( A e. X /\ B e. X /\ C e. X ) ) -> ( A D B ) <_ ( ( A D C ) + ( B D C ) ) )
Theorem	xmetrtri 14612	One half of the reverse triangle inequality for the distance function of an extended metric. (Contributed by Mario Carneiro, 4-Sep-2015.)
|- ( ( D e. ( *Met ` X ) /\ ( A e. X /\ B e. X /\ C e. X ) ) -> ( ( A D C ) +e -e ( B D C ) ) <_ ( A D B ) )
Theorem	metrtri 14613	Reverse triangle inequality for the distance function of a metric space. (Contributed by Mario Carneiro, 5-May-2014.) (Revised by Jim Kingdon, 21-Apr-2023.)
|- ( ( D e. ( Met ` X ) /\ ( A e. X /\ B e. X /\ C e. X ) ) -> ( abs ` ( ( A D C ) - ( B D C ) ) ) <_ ( A D B ) )
Theorem	metn0 14614	A metric space is nonempty iff its base set is nonempty. (Contributed by NM, 4-Oct-2007.) (Revised by Mario Carneiro, 14-Aug-2015.)
|- ( D e. ( Met ` X ) -> ( D =/= (/) <-> X =/= (/) ) )
Theorem	xmetres2 14615	Restriction of an extended metric. (Contributed by Mario Carneiro, 20-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ R C_ X ) -> ( D |` ( R X. R ) ) e. ( *Met ` R ) )
Theorem	metreslem 14616	Lemma for metres 14619. (Contributed by Mario Carneiro, 24-Aug-2015.)
|- ( dom D = ( X X. X ) -> ( D |` ( R X. R ) ) = ( D |` ( ( X i^i R ) X. ( X i^i R ) ) ) )
Theorem	metres2 14617	Lemma for metres 14619. (Contributed by FL, 12-Oct-2006.) (Proof shortened by Mario Carneiro, 14-Aug-2015.)
|- ( ( D e. ( Met ` X ) /\ R C_ X ) -> ( D |` ( R X. R ) ) e. ( Met ` R ) )
Theorem	xmetres 14618	A restriction of an extended metric is an extended metric. (Contributed by Mario Carneiro, 24-Aug-2015.)
|- ( D e. ( *Met ` X ) -> ( D |` ( R X. R ) ) e. ( *Met ` ( X i^i R ) ) )
Theorem	metres 14619	A restriction of a metric is a metric. (Contributed by NM, 26-Aug-2007.) (Revised by Mario Carneiro, 14-Aug-2015.)
|- ( D e. ( Met ` X ) -> ( D |` ( R X. R ) ) e. ( Met ` ( X i^i R ) ) )
Theorem	0met 14620	The empty metric. (Contributed by NM, 30-Aug-2006.) (Revised by Mario Carneiro, 14-Aug-2015.)
|- (/) e. ( Met ` (/) )
9.2.3  Metric space balls
Theorem	blfvalps 14621*	The value of the ball function. (Contributed by NM, 30-Aug-2006.) (Revised by Mario Carneiro, 11-Nov-2013.) (Revised by Thierry Arnoux, 11-Feb-2018.)
|- ( D e. (PsMet ` X ) -> ( ball ` D ) = ( x e. X , r e. RR* |-> { y e. X | ( x D y ) < r } ) )
Theorem	blfval 14622*	The value of the ball function. (Contributed by NM, 30-Aug-2006.) (Revised by Mario Carneiro, 11-Nov-2013.) (Proof shortened by Thierry Arnoux, 11-Feb-2018.)
|- ( D e. ( *Met ` X ) -> ( ball ` D ) = ( x e. X , r e. RR* |-> { y e. X | ( x D y ) < r } ) )
Theorem	blex 14623	A ball is a set. Also see blfn 14107 in case you just know D is a set, not D e. ( *Met ` X ). (Contributed by Jim Kingdon, 4-May-2023.)
|- ( D e. ( *Met ` X ) -> ( ball ` D ) e. _V )
Theorem	blvalps 14624*	The ball around a point P is the set of all points whose distance from P is less than the ball's radius R. (Contributed by NM, 31-Aug-2006.) (Revised by Mario Carneiro, 11-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ P e. X /\ R e. RR* ) -> ( P ( ball ` D ) R ) = { x e. X | ( P D x ) < R } )
Theorem	blval 14625*	The ball around a point P is the set of all points whose distance from P is less than the ball's radius R. (Contributed by NM, 31-Aug-2006.) (Revised by Mario Carneiro, 11-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR* ) -> ( P ( ball ` D ) R ) = { x e. X | ( P D x ) < R } )
Theorem	elblps 14626	Membership in a ball. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 11-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ P e. X /\ R e. RR* ) -> ( A e. ( P ( ball ` D ) R ) <-> ( A e. X /\ ( P D A ) < R ) ) )
Theorem	elbl 14627	Membership in a ball. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 11-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR* ) -> ( A e. ( P ( ball ` D ) R ) <-> ( A e. X /\ ( P D A ) < R ) ) )
Theorem	elbl2ps 14628	Membership in a ball. (Contributed by NM, 9-Mar-2007.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( ( D e. (PsMet ` X ) /\ R e. RR* ) /\ ( P e. X /\ A e. X ) ) -> ( A e. ( P ( ball ` D ) R ) <-> ( P D A ) < R ) )
Theorem	elbl2 14629	Membership in a ball. (Contributed by NM, 9-Mar-2007.)
|- ( ( ( D e. ( *Met ` X ) /\ R e. RR* ) /\ ( P e. X /\ A e. X ) ) -> ( A e. ( P ( ball ` D ) R ) <-> ( P D A ) < R ) )
Theorem	elbl3ps 14630	Membership in a ball, with reversed distance function arguments. (Contributed by NM, 10-Nov-2007.)
|- ( ( ( D e. (PsMet ` X ) /\ R e. RR* ) /\ ( P e. X /\ A e. X ) ) -> ( A e. ( P ( ball ` D ) R ) <-> ( A D P ) < R ) )
Theorem	elbl3 14631	Membership in a ball, with reversed distance function arguments. (Contributed by NM, 10-Nov-2007.)
|- ( ( ( D e. ( *Met ` X ) /\ R e. RR* ) /\ ( P e. X /\ A e. X ) ) -> ( A e. ( P ( ball ` D ) R ) <-> ( A D P ) < R ) )
Theorem	blcomps 14632	Commute the arguments to the ball function. (Contributed by Mario Carneiro, 22-Jan-2014.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( ( D e. (PsMet ` X ) /\ R e. RR* ) /\ ( P e. X /\ A e. X ) ) -> ( A e. ( P ( ball ` D ) R ) <-> P e. ( A ( ball ` D ) R ) ) )
Theorem	blcom 14633	Commute the arguments to the ball function. (Contributed by Mario Carneiro, 22-Jan-2014.)
|- ( ( ( D e. ( *Met ` X ) /\ R e. RR* ) /\ ( P e. X /\ A e. X ) ) -> ( A e. ( P ( ball ` D ) R ) <-> P e. ( A ( ball ` D ) R ) ) )
Theorem	xblpnfps 14634	The infinity ball in an extended metric is the set of all points that are a finite distance from the center. (Contributed by Mario Carneiro, 23-Aug-2015.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ P e. X ) -> ( A e. ( P ( ball ` D ) +oo ) <-> ( A e. X /\ ( P D A ) e. RR ) ) )
Theorem	xblpnf 14635	The infinity ball in an extended metric is the set of all points that are a finite distance from the center. (Contributed by Mario Carneiro, 23-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ P e. X ) -> ( A e. ( P ( ball ` D ) +oo ) <-> ( A e. X /\ ( P D A ) e. RR ) ) )
Theorem	blpnf 14636	The infinity ball in a standard metric is just the whole space. (Contributed by Mario Carneiro, 23-Aug-2015.)
|- ( ( D e. ( Met ` X ) /\ P e. X ) -> ( P ( ball ` D ) +oo ) = X )
Theorem	bldisj 14637	Two balls are disjoint if the center-to-center distance is more than the sum of the radii. (Contributed by Mario Carneiro, 30-Dec-2013.)
|- ( ( ( D e. ( *Met ` X ) /\ P e. X /\ Q e. X ) /\ ( R e. RR* /\ S e. RR* /\ ( R +e S ) <_ ( P D Q ) ) ) -> ( ( P ( ball ` D ) R ) i^i ( Q ( ball ` D ) S ) ) = (/) )
Theorem	blgt0 14638	A nonempty ball implies that the radius is positive. (Contributed by NM, 11-Mar-2007.) (Revised by Mario Carneiro, 23-Aug-2015.)
|- ( ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR* ) /\ A e. ( P ( ball ` D ) R ) ) -> 0 < R )
Theorem	bl2in 14639	Two balls are disjoint if they don't overlap. (Contributed by NM, 11-Mar-2007.) (Revised by Mario Carneiro, 23-Aug-2015.)
|- ( ( ( D e. ( Met ` X ) /\ P e. X /\ Q e. X ) /\ ( R e. RR /\ R <_ ( ( P D Q ) / 2 ) ) ) -> ( ( P ( ball ` D ) R ) i^i ( Q ( ball ` D ) R ) ) = (/) )
Theorem	xblss2ps 14640	One ball is contained in another if the center-to-center distance is less than the difference of the radii. In this version of blss2 14643 for extended metrics, we have to assume the balls are a finite distance apart, or else P will not even be in the infinity ball around Q. (Contributed by Mario Carneiro, 23-Aug-2015.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ph -> D e. (PsMet ` X ) )   &    |- ( ph -> P e. X )   &    |- ( ph -> Q e. X )   &    |- ( ph -> R e. RR* )   &    |- ( ph -> S e. RR* )   &    |- ( ph -> ( P D Q ) e. RR )   &    |- ( ph -> ( P D Q ) <_ ( S +e -e R ) )   =>    |- ( ph -> ( P ( ball ` D ) R ) C_ ( Q ( ball ` D ) S ) )
Theorem	xblss2 14641	One ball is contained in another if the center-to-center distance is less than the difference of the radii. In this version of blss2 14643 for extended metrics, we have to assume the balls are a finite distance apart, or else P will not even be in the infinity ball around Q. (Contributed by Mario Carneiro, 23-Aug-2015.)
|- ( ph -> D e. ( *Met ` X ) )   &    |- ( ph -> P e. X )   &    |- ( ph -> Q e. X )   &    |- ( ph -> R e. RR* )   &    |- ( ph -> S e. RR* )   &    |- ( ph -> ( P D Q ) e. RR )   &    |- ( ph -> ( P D Q ) <_ ( S +e -e R ) )   =>    |- ( ph -> ( P ( ball ` D ) R ) C_ ( Q ( ball ` D ) S ) )
Theorem	blss2ps 14642	One ball is contained in another if the center-to-center distance is less than the difference of the radii. (Contributed by Mario Carneiro, 15-Jan-2014.) (Revised by Mario Carneiro, 23-Aug-2015.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( ( D e. (PsMet ` X ) /\ P e. X /\ Q e. X ) /\ ( R e. RR /\ S e. RR /\ ( P D Q ) <_ ( S - R ) ) ) -> ( P ( ball ` D ) R ) C_ ( Q ( ball ` D ) S ) )
Theorem	blss2 14643	One ball is contained in another if the center-to-center distance is less than the difference of the radii. (Contributed by Mario Carneiro, 15-Jan-2014.) (Revised by Mario Carneiro, 23-Aug-2015.)
|- ( ( ( D e. ( *Met ` X ) /\ P e. X /\ Q e. X ) /\ ( R e. RR /\ S e. RR /\ ( P D Q ) <_ ( S - R ) ) ) -> ( P ( ball ` D ) R ) C_ ( Q ( ball ` D ) S ) )
Theorem	blhalf 14644	A ball of radius R / 2 is contained in a ball of radius R centered at any point inside the smaller ball. (Contributed by Jeff Madsen, 2-Sep-2009.) (Proof shortened by Mario Carneiro, 14-Jan-2014.)
|- ( ( ( M e. ( *Met ` X ) /\ Y e. X ) /\ ( R e. RR /\ Z e. ( Y ( ball ` M ) ( R / 2 ) ) ) ) -> ( Y ( ball ` M ) ( R / 2 ) ) C_ ( Z ( ball ` M ) R ) )
Theorem	blfps 14645	Mapping of a ball. (Contributed by NM, 7-May-2007.) (Revised by Mario Carneiro, 23-Aug-2015.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( D e. (PsMet ` X ) -> ( ball ` D ) : ( X X. RR* ) --> ~P X )
Theorem	blf 14646	Mapping of a ball. (Contributed by NM, 7-May-2007.) (Revised by Mario Carneiro, 23-Aug-2015.)
|- ( D e. ( *Met ` X ) -> ( ball ` D ) : ( X X. RR* ) --> ~P X )
Theorem	blrnps 14647*	Membership in the range of the ball function. Note that ran ( ball ` D ) is the collection of all balls for metric D. (Contributed by NM, 31-Aug-2006.) (Revised by Mario Carneiro, 12-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( D e. (PsMet ` X ) -> ( A e. ran ( ball ` D ) <-> E. x e. X E. r e. RR* A = ( x ( ball ` D ) r ) ) )
Theorem	blrn 14648*	Membership in the range of the ball function. Note that ran ( ball ` D ) is the collection of all balls for metric D. (Contributed by NM, 31-Aug-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( D e. ( *Met ` X ) -> ( A e. ran ( ball ` D ) <-> E. x e. X E. r e. RR* A = ( x ( ball ` D ) r ) ) )
Theorem	xblcntrps 14649	A ball contains its center. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ P e. X /\ ( R e. RR* /\ 0 < R ) ) -> P e. ( P ( ball ` D ) R ) )
Theorem	xblcntr 14650	A ball contains its center. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ ( R e. RR* /\ 0 < R ) ) -> P e. ( P ( ball ` D ) R ) )
Theorem	blcntrps 14651	A ball contains its center. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ P e. X /\ R e. RR+ ) -> P e. ( P ( ball ` D ) R ) )
Theorem	blcntr 14652	A ball contains its center. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR+ ) -> P e. ( P ( ball ` D ) R ) )
Theorem	xblm 14653*	A ball is inhabited iff the radius is positive. (Contributed by Mario Carneiro, 23-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR* ) -> ( E. x x e. ( P ( ball ` D ) R ) <-> 0 < R ) )
Theorem	bln0 14654	A ball is not empty. It is also inhabited, as seen at blcntr 14652. (Contributed by NM, 6-Oct-2007.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR+ ) -> ( P ( ball ` D ) R ) =/= (/) )
Theorem	blelrnps 14655	A ball belongs to the set of balls of a metric space. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ P e. X /\ R e. RR* ) -> ( P ( ball ` D ) R ) e. ran ( ball ` D ) )
Theorem	blelrn 14656	A ball belongs to the set of balls of a metric space. (Contributed by NM, 2-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR* ) -> ( P ( ball ` D ) R ) e. ran ( ball ` D ) )
Theorem	blssm 14657	A ball is a subset of the base set of a metric space. (Contributed by NM, 31-Aug-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR* ) -> ( P ( ball ` D ) R ) C_ X )
Theorem	unirnblps 14658	The union of the set of balls of a metric space is its base set. (Contributed by NM, 12-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( D e. (PsMet ` X ) -> U. ran ( ball ` D ) = X )
Theorem	unirnbl 14659	The union of the set of balls of a metric space is its base set. (Contributed by NM, 12-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( D e. ( *Met ` X ) -> U. ran ( ball ` D ) = X )
Theorem	blininf 14660	The intersection of two balls with the same center is the smaller of them. (Contributed by NM, 1-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( ( D e. ( *Met ` X ) /\ P e. X ) /\ ( R e. RR* /\ S e. RR* ) ) -> ( ( P ( ball ` D ) R ) i^i ( P ( ball ` D ) S ) ) = ( P ( ball ` D )inf ( { R , S } , RR* , < ) ) )
Theorem	ssblps 14661	The size of a ball increases monotonically with its radius. (Contributed by NM, 20-Sep-2007.) (Revised by Mario Carneiro, 24-Aug-2015.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( ( D e. (PsMet ` X ) /\ P e. X ) /\ ( R e. RR* /\ S e. RR* ) /\ R <_ S ) -> ( P ( ball ` D ) R ) C_ ( P ( ball ` D ) S ) )
Theorem	ssbl 14662	The size of a ball increases monotonically with its radius. (Contributed by NM, 20-Sep-2007.) (Revised by Mario Carneiro, 24-Aug-2015.)
|- ( ( ( D e. ( *Met ` X ) /\ P e. X ) /\ ( R e. RR* /\ S e. RR* ) /\ R <_ S ) -> ( P ( ball ` D ) R ) C_ ( P ( ball ` D ) S ) )
Theorem	blssps 14663*	Any point P in a ball B can be centered in another ball that is a subset of B. (Contributed by NM, 31-Aug-2006.) (Revised by Mario Carneiro, 24-Aug-2015.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ B e. ran ( ball ` D ) /\ P e. B ) -> E. x e. RR+ ( P ( ball ` D ) x ) C_ B )
Theorem	blss 14664*	Any point P in a ball B can be centered in another ball that is a subset of B. (Contributed by NM, 31-Aug-2006.) (Revised by Mario Carneiro, 24-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ B e. ran ( ball ` D ) /\ P e. B ) -> E. x e. RR+ ( P ( ball ` D ) x ) C_ B )
Theorem	blssexps 14665*	Two ways to express the existence of a ball subset. (Contributed by NM, 5-May-2007.) (Revised by Mario Carneiro, 12-Nov-2013.) (Revised by Thierry Arnoux, 11-Mar-2018.)
|- ( ( D e. (PsMet ` X ) /\ P e. X ) -> ( E. x e. ran ( ball ` D ) ( P e. x /\ x C_ A ) <-> E. r e. RR+ ( P ( ball ` D ) r ) C_ A ) )
Theorem	blssex 14666*	Two ways to express the existence of a ball subset. (Contributed by NM, 5-May-2007.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( D e. ( *Met ` X ) /\ P e. X ) -> ( E. x e. ran ( ball ` D ) ( P e. x /\ x C_ A ) <-> E. r e. RR+ ( P ( ball ` D ) r ) C_ A ) )
Theorem	ssblex 14667*	A nested ball exists whose radius is less than any desired amount. (Contributed by NM, 20-Sep-2007.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- ( ( ( D e. ( *Met ` X ) /\ P e. X ) /\ ( R e. RR+ /\ S e. RR+ ) ) -> E. x e. RR+ ( x < R /\ ( P ( ball ` D ) x ) C_ ( P ( ball ` D ) S ) ) )
Theorem	blin2 14668*	Given any two balls and a point in their intersection, there is a ball contained in the intersection with the given center point. (Contributed by Mario Carneiro, 12-Nov-2013.)
|- ( ( ( D e. ( *Met ` X ) /\ P e. ( B i^i C ) ) /\ ( B e. ran ( ball ` D ) /\ C e. ran ( ball ` D ) ) ) -> E. x e. RR+ ( P ( ball ` D ) x ) C_ ( B i^i C ) )
Theorem	blbas 14669	The balls of a metric space form a basis for a topology. (Contributed by NM, 12-Sep-2006.) (Revised by Mario Carneiro, 15-Jan-2014.)
|- ( D e. ( *Met ` X ) -> ran ( ball ` D ) e. TopBases )
Theorem	blres 14670	A ball in a restricted metric space. (Contributed by Mario Carneiro, 5-Jan-2014.)
|- C = ( D |` ( Y X. Y ) )   =>    |- ( ( D e. ( *Met ` X ) /\ P e. ( X i^i Y ) /\ R e. RR* ) -> ( P ( ball ` C ) R ) = ( ( P ( ball ` D ) R ) i^i Y ) )
Theorem	xmeterval 14671	Value of the "finitely separated" relation. (Contributed by Mario Carneiro, 24-Aug-2015.)
|- .~ = ( `' D " RR )   =>    |- ( D e. ( *Met ` X ) -> ( A .~ B <-> ( A e. X /\ B e. X /\ ( A D B ) e. RR ) ) )
Theorem	xmeter 14672	The "finitely separated" relation is an equivalence relation. (Contributed by Mario Carneiro, 24-Aug-2015.)
|- .~ = ( `' D " RR )   =>    |- ( D e. ( *Met ` X ) -> .~ Er X )
Theorem	xmetec 14673	The equivalence classes under the finite separation equivalence relation are infinity balls. (Contributed by Mario Carneiro, 24-Aug-2015.)
|- .~ = ( `' D " RR )   =>    |- ( ( D e. ( *Met ` X ) /\ P e. X ) -> [ P ] .~ = ( P ( ball ` D ) +oo ) )
Theorem	blssec 14674	A ball centered at P is contained in the set of points finitely separated from P. This is just an application of ssbl 14662 to the infinity ball. (Contributed by Mario Carneiro, 24-Aug-2015.)
|- .~ = ( `' D " RR )   =>    |- ( ( D e. ( *Met ` X ) /\ P e. X /\ S e. RR* ) -> ( P ( ball ` D ) S ) C_ [ P ] .~ )
Theorem	blpnfctr 14675	The infinity ball in an extended metric acts like an ultrametric ball in that every point in the ball is also its center. (Contributed by Mario Carneiro, 21-Aug-2015.)
|- ( ( D e. ( *Met ` X ) /\ P e. X /\ A e. ( P ( ball ` D ) +oo ) ) -> ( P ( ball ` D ) +oo ) = ( A ( ball ` D ) +oo ) )
Theorem	xmetresbl 14676	An extended metric restricted to any ball (in particular the infinity ball) is a proper metric. Together with xmetec 14673, this shows that any extended metric space can be "factored" into the disjoint union of proper metric spaces, with points in the same region measured by that region's metric, and points in different regions being distance +oo from each other. (Contributed by Mario Carneiro, 23-Aug-2015.)
|- B = ( P ( ball ` D ) R )   =>    |- ( ( D e. ( *Met ` X ) /\ P e. X /\ R e. RR* ) -> ( D |` ( B X. B ) ) e. ( Met ` B ) )
9.2.4  Open sets of a metric space
Theorem	mopnrel 14677	The class of open sets of a metric space is a relation. (Contributed by Jim Kingdon, 5-May-2023.)
|- Rel MetOpen
Theorem	mopnval 14678	An open set is a subset of a metric space which includes a ball around each of its points. Definition 1.3-2 of [Kreyszig] p. 18. The object ( MetOpen ` D ) is the family of all open sets in the metric space determined by the metric D. By mopntop 14680, the open sets of a metric space form a topology J, whose base set is U. J by mopnuni 14681. (Contributed by NM, 1-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> J = ( topGen ` ran ( ball ` D ) ) )
Theorem	mopntopon 14679	The set of open sets of a metric space X is a topology on X. Remark in [Kreyszig] p. 19. This theorem connects the two concepts and makes available the theorems for topologies for use with metric spaces. (Contributed by Mario Carneiro, 24-Aug-2015.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> J e. (TopOn ` X ) )
Theorem	mopntop 14680	The set of open sets of a metric space is a topology. (Contributed by NM, 28-Aug-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> J e. Top )
Theorem	mopnuni 14681	The union of all open sets in a metric space is its underlying set. (Contributed by NM, 4-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> X = U. J )
Theorem	elmopn 14682*	The defining property of an open set of a metric space. (Contributed by NM, 1-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> ( A e. J <-> ( A C_ X /\ A. x e. A E. y e. ran ( ball ` D ) ( x e. y /\ y C_ A ) ) ) )
Theorem	mopnfss 14683	The family of open sets of a metric space is a collection of subsets of the base set. (Contributed by NM, 3-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> J C_ ~P X )
Theorem	mopnm 14684	The base set of a metric space is open. Part of Theorem T1 of [Kreyszig] p. 19. (Contributed by NM, 4-Sep-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> X e. J )
Theorem	elmopn2 14685*	A defining property of an open set of a metric space. (Contributed by NM, 5-May-2007.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- J = ( MetOpen ` D )   =>    |- ( D e. ( *Met ` X ) -> ( A e. J <-> ( A C_ X /\ A. x e. A E. y e. RR+ ( x ( ball ` D ) y ) C_ A ) ) )
Theorem	mopnss 14686	An open set of a metric space is a subspace of its base set. (Contributed by NM, 3-Sep-2006.)
|- J = ( MetOpen ` D )   =>    |- ( ( D e. ( *Met ` X ) /\ A e. J ) -> A C_ X )
Theorem	isxms 14687	Express the predicate " <. X , D >. is an extended metric space" with underlying set X and distance function D. (Contributed by Mario Carneiro, 2-Sep-2015.)
|- J = ( TopOpen ` K )   &    |- X = ( Base ` K )   &    |- D = ( ( dist ` K ) |` ( X X. X ) )   =>    |- ( K e. *MetSp <-> ( K e. TopSp /\ J = ( MetOpen ` D ) ) )
Theorem	isxms2 14688	Express the predicate " <. X , D >. is an extended metric space" with underlying set X and distance function D. (Contributed by Mario Carneiro, 2-Sep-2015.)
|- J = ( TopOpen ` K )   &    |- X = ( Base ` K )   &    |- D = ( ( dist ` K ) |` ( X X. X ) )   =>    |- ( K e. *MetSp <-> ( D e. ( *Met ` X ) /\ J = ( MetOpen ` D ) ) )
Theorem	isms 14689	Express the predicate " <. X , D >. is a metric space" with underlying set X and distance function D. (Contributed by NM, 27-Aug-2006.) (Revised by Mario Carneiro, 24-Aug-2015.)
|- J = ( TopOpen ` K )   &    |- X = ( Base ` K )   &    |- D = ( ( dist ` K ) |` ( X X. X ) )   =>    |- ( K e. MetSp <-> ( K e. *MetSp /\ D e. ( Met ` X ) ) )
Theorem	isms2 14690	Express the predicate " <. X , D >. is a metric space" with underlying set X and distance function D. (Contributed by NM, 27-Aug-2006.) (Revised by Mario Carneiro, 24-Aug-2015.)
|- J = ( TopOpen ` K )   &    |- X = ( Base ` K )   &    |- D = ( ( dist ` K ) |` ( X X. X ) )   =>    |- ( K e. MetSp <-> ( D e. ( Met ` X ) /\ J = ( MetOpen ` D ) ) )
Theorem	xmstopn 14691	The topology component of an extended metric space coincides with the topology generated by the metric component. (Contributed by Mario Carneiro, 26-Aug-2015.)
|- J = ( TopOpen ` K )   &    |- X = ( Base ` K )   &    |- D = ( ( dist ` K ) |` ( X X. X ) )   =>    |- ( K e. *MetSp -> J = ( MetOpen ` D ) )
Theorem	mstopn 14692	The topology component of a metric space coincides with the topology generated by the metric component. (Contributed by Mario Carneiro, 26-Aug-2015.)
|- J = ( TopOpen ` K )   &    |- X = ( Base ` K )   &    |- D = ( ( dist ` K ) |` ( X X. X ) )   =>    |- ( K e. MetSp -> J = ( MetOpen ` D ) )
Theorem	xmstps 14693	An extended metric space is a topological space. (Contributed by Mario Carneiro, 26-Aug-2015.)
|- ( M e. *MetSp -> M e. TopSp )
Theorem	msxms 14694	A metric space is an extended metric space. (Contributed by Mario Carneiro, 26-Aug-2015.)
|- ( M e. MetSp -> M e. *MetSp )
Theorem	mstps 14695	A metric space is a topological space. (Contributed by Mario Carneiro, 26-Aug-2015.)
|- ( M e. MetSp -> M e. TopSp )
Theorem	xmsxmet 14696	The distance function, suitably truncated, is an extended metric on X. (Contributed by Mario Carneiro, 2-Sep-2015.)
|- X = ( Base ` M )   &    |- D = ( ( dist ` M ) |` ( X X. X ) )   =>    |- ( M e. *MetSp -> D e. ( *Met ` X ) )
Theorem	msmet 14697	The distance function, suitably truncated, is a metric on X. (Contributed by Mario Carneiro, 12-Nov-2013.)
|- X = ( Base ` M )   &    |- D = ( ( dist ` M ) |` ( X X. X ) )   =>    |- ( M e. MetSp -> D e. ( Met ` X ) )
Theorem	msf 14698	The distance function of a metric space is a function into the real numbers. (Contributed by NM, 30-Aug-2006.) (Revised by Mario Carneiro, 12-Nov-2013.)
|- X = ( Base ` M )   &    |- D = ( ( dist ` M ) |` ( X X. X ) )   =>    |- ( M e. MetSp -> D : ( X X. X ) --> RR )
Theorem	xmsxmet2 14699	The distance function, suitably truncated, is an extended metric on X. (Contributed by Mario Carneiro, 2-Oct-2015.)
|- X = ( Base ` M )   &    |- D = ( dist ` M )   =>    |- ( M e. *MetSp -> ( D |` ( X X. X ) ) e. ( *Met ` X ) )
Theorem	msmet2 14700	The distance function, suitably truncated, is a metric on X. (Contributed by Mario Carneiro, 2-Oct-2015.)
|- X = ( Base ` M )   &    |- D = ( dist ` M )   =>    |- ( M e. MetSp -> ( D |` ( X X. X ) ) e. ( Met ` X ) )