P. 9 of Theorem List - Intuitionistic Logic Explorer

Theorem List for Intuitionistic Logic Explorer - 801-900   *Has distinct variable group(s)

Type	Label	Description
Statement
Theorem	pm2.73 801	Theorem *2.73 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.)
|- ( ( ph -> ps ) -> ( ( ( ph \/ ps ) \/ ch ) -> ( ps \/ ch ) ) )
Theorem	pm2.74 802	Theorem *2.74 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Mario Carneiro, 31-Jan-2015.)
|- ( ( ps -> ph ) -> ( ( ( ph \/ ps ) \/ ch ) -> ( ph \/ ch ) ) )
Theorem	pm2.76 803	Theorem *2.76 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.) (Revised by Mario Carneiro, 31-Jan-2015.)
|- ( ( ph \/ ( ps -> ch ) ) -> ( ( ph \/ ps ) -> ( ph \/ ch ) ) )
Theorem	pm2.75 804	Theorem *2.75 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Wolf Lammen, 4-Jan-2013.)
|- ( ( ph \/ ps ) -> ( ( ph \/ ( ps -> ch ) ) -> ( ph \/ ch ) ) )
Theorem	pm2.8 805	Theorem *2.8 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.) (Proof shortened by Mario Carneiro, 31-Jan-2015.)
|- ( ( ph \/ ps ) -> ( ( -. ps \/ ch ) -> ( ph \/ ch ) ) )
Theorem	pm2.81 806	Theorem *2.81 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.)
|- ( ( ps -> ( ch -> th ) ) -> ( ( ph \/ ps ) -> ( ( ph \/ ch ) -> ( ph \/ th ) ) ) )
Theorem	pm2.82 807	Theorem *2.82 of [WhiteheadRussell] p. 108. (Contributed by NM, 3-Jan-2005.)
|- ( ( ( ph \/ ps ) \/ ch ) -> ( ( ( ph \/ -. ch ) \/ th ) -> ( ( ph \/ ps ) \/ th ) ) )
Theorem	pm3.2ni 808	Infer negated disjunction of negated premises. (Contributed by NM, 4-Apr-1995.)
|- -. ph   &    |- -. ps   =>    |- -. ( ph \/ ps )
Theorem	orabs 809	Absorption of redundant internal disjunct. Compare Theorem *4.45 of [WhiteheadRussell] p. 119. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 28-Feb-2014.)
|- ( ph <-> ( ( ph \/ ps ) /\ ph ) )
Theorem	oranabs 810	Absorb a disjunct into a conjunct. (Contributed by Roy F. Longton, 23-Jun-2005.) (Proof shortened by Wolf Lammen, 10-Nov-2013.)
|- ( ( ( ph \/ -. ps ) /\ ps ) <-> ( ph /\ ps ) )
Theorem	ordi 811	Distributive law for disjunction. Theorem *4.41 of [WhiteheadRussell] p. 119. (Contributed by NM, 5-Aug-1993.) (Revised by Mario Carneiro, 31-Jan-2015.)
|- ( ( ph \/ ( ps /\ ch ) ) <-> ( ( ph \/ ps ) /\ ( ph \/ ch ) ) )
Theorem	ordir 812	Distributive law for disjunction. (Contributed by NM, 12-Aug-1994.)
|- ( ( ( ph /\ ps ) \/ ch ) <-> ( ( ph \/ ch ) /\ ( ps \/ ch ) ) )
Theorem	andi 813	Distributive law for conjunction. Theorem *4.4 of [WhiteheadRussell] p. 118. (Contributed by NM, 5-Aug-1993.) (Proof shortened by Wolf Lammen, 5-Jan-2013.)
|- ( ( ph /\ ( ps \/ ch ) ) <-> ( ( ph /\ ps ) \/ ( ph /\ ch ) ) )
Theorem	andir 814	Distributive law for conjunction. (Contributed by NM, 12-Aug-1994.)
|- ( ( ( ph \/ ps ) /\ ch ) <-> ( ( ph /\ ch ) \/ ( ps /\ ch ) ) )
Theorem	orddi 815	Double distributive law for disjunction. (Contributed by NM, 12-Aug-1994.)
|- ( ( ( ph /\ ps ) \/ ( ch /\ th ) ) <-> ( ( ( ph \/ ch ) /\ ( ph \/ th ) ) /\ ( ( ps \/ ch ) /\ ( ps \/ th ) ) ) )
Theorem	anddi 816	Double distributive law for conjunction. (Contributed by NM, 12-Aug-1994.)
|- ( ( ( ph \/ ps ) /\ ( ch \/ th ) ) <-> ( ( ( ph /\ ch ) \/ ( ph /\ th ) ) \/ ( ( ps /\ ch ) \/ ( ps /\ th ) ) ) )
Theorem	pm4.39 817	Theorem *4.39 of [WhiteheadRussell] p. 118. (Contributed by NM, 3-Jan-2005.)
|- ( ( ( ph <-> ch ) /\ ( ps <-> th ) ) -> ( ( ph \/ ps ) <-> ( ch \/ th ) ) )
Theorem	animorl 818	Conjunction implies disjunction with one common formula (1/4). (Contributed by BJ, 4-Oct-2019.)
|- ( ( ph /\ ps ) -> ( ph \/ ch ) )
Theorem	animorr 819	Conjunction implies disjunction with one common formula (2/4). (Contributed by BJ, 4-Oct-2019.)
|- ( ( ph /\ ps ) -> ( ch \/ ps ) )
Theorem	animorlr 820	Conjunction implies disjunction with one common formula (3/4). (Contributed by BJ, 4-Oct-2019.)
|- ( ( ph /\ ps ) -> ( ch \/ ph ) )
Theorem	animorrl 821	Conjunction implies disjunction with one common formula (4/4). (Contributed by BJ, 4-Oct-2019.)
|- ( ( ph /\ ps ) -> ( ps \/ ch ) )
Theorem	pm4.72 822	Implication in terms of biconditional and disjunction. Theorem *4.72 of [WhiteheadRussell] p. 121. (Contributed by NM, 30-Aug-1993.) (Proof shortened by Wolf Lammen, 30-Jan-2013.)
|- ( ( ph -> ps ) <-> ( ps <-> ( ph \/ ps ) ) )
Theorem	pm5.16 823	Theorem *5.16 of [WhiteheadRussell] p. 124. (Contributed by NM, 3-Jan-2005.) (Revised by Mario Carneiro, 31-Jan-2015.)
|- -. ( ( ph <-> ps ) /\ ( ph <-> -. ps ) )
Theorem	biort 824	A disjunction with a true formula is equivalent to that true formula. (Contributed by NM, 23-May-1999.)
|- ( ph -> ( ph <-> ( ph \/ ps ) ) )
1.2.7  Stable propositions
Syntax	wstab 825	Extend wff definition to include stability.
wff STAB ph
Definition	df-stab 826	Propositions where a double-negative can be removed are called stable. See Chapter 2 [Moschovakis] p. 2. Our notation for stability is a connective STAB which we place before the formula in question. For example, STAB x = y corresponds to " x = y is stable". (Contributed by David A. Wheeler, 13-Aug-2018.)
|- (STAB ph <-> ( -. -. ph -> ph ) )
Theorem	stbid 827	The equivalent of a stable proposition is stable. (Contributed by Jim Kingdon, 12-Aug-2022.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> (STAB ps <-> STAB ch ) )
Theorem	stabnot 828	Every negated formula is stable. (Contributed by David A. Wheeler, 13-Aug-2018.)
|- STAB -. ph
1.2.8  Decidable propositions
Syntax	wdc 829	Extend wff definition to include decidability.
wff DECID ph
Definition	df-dc 830	Propositions which are known to be true or false are called decidable. The (classical) Law of the Excluded Middle corresponds to the principle that all propositions are decidable, but even given intuitionistic logic, particular kinds of propositions may be decidable (for example, the proposition that two natural numbers are equal will be decidable under most sets of axioms). Our notation for decidability is a connective DECID which we place before the formula in question. For example, DECID x = y corresponds to " x = y is decidable". We could transform intuitionistic logic to classical logic by adding unconditional forms of condc 848, exmiddc 831, peircedc 909, or notnotrdc 838, any of which would correspond to the assertion that all propositions are decidable. (Contributed by Jim Kingdon, 11-Mar-2018.)
|- (DECID ph <-> ( ph \/ -. ph ) )
Theorem	exmiddc 831	Law of excluded middle, for a decidable proposition. The law of the excluded middle is also called the principle of tertium non datur. Theorem *2.11 of [WhiteheadRussell] p. 101. It says that something is either true or not true; there are no in-between values of truth. The key way in which intuitionistic logic differs from classical logic is that intuitionistic logic says that excluded middle only holds for some propositions, and classical logic says that it holds for all propositions. (Contributed by Jim Kingdon, 12-May-2018.)
|- (DECID ph -> ( ph \/ -. ph ) )
Theorem	pm2.1dc 832	Commuted law of the excluded middle for a decidable proposition. Based on theorem *2.1 of [WhiteheadRussell] p. 101. (Contributed by Jim Kingdon, 25-Mar-2018.)
|- (DECID ph -> ( -. ph \/ ph ) )
Theorem	dcbid 833	Equivalence property for decidability. Deduction form. (Contributed by Jim Kingdon, 7-Sep-2019.)
|- ( ph -> ( ps <-> ch ) )   =>    |- ( ph -> (DECID ps <-> DECID ch ) )
Theorem	dcbiit 834	Equivalence property for decidability. Closed form. (Contributed by BJ, 27-Jan-2020.)
|- ( ( ph <-> ps ) -> (DECID ph <-> DECID ps ) )
Theorem	dcbii 835	Equivalence property for decidability. Inference form. (Contributed by Jim Kingdon, 28-Mar-2018.)
|- ( ph <-> ps )   =>    |- (DECID ph <-> DECID ps )
Theorem	dcim 836	An implication between two decidable propositions is decidable. (Contributed by Jim Kingdon, 28-Mar-2018.)
|- (DECID ph -> (DECID ps -> DECID ( ph -> ps ) ) )
Theorem	dcn 837	The negation of a decidable proposition is decidable. The converse need not hold, but does hold for negated propositions, see dcnn 843. (Contributed by Jim Kingdon, 25-Mar-2018.)
|- (DECID ph -> DECID -. ph )
Theorem	notnotrdc 838	Double negation elimination for a decidable proposition. The converse, notnot 624, holds for all propositions, not just decidable ones. This is Theorem *2.14 of [WhiteheadRussell] p. 102, but with a decidability condition added. (Contributed by Jim Kingdon, 11-Mar-2018.)
|- (DECID ph -> ( -. -. ph -> ph ) )
Theorem	dcstab 839	Decidability implies stability. The converse need not hold. (Contributed by David A. Wheeler, 13-Aug-2018.)
|- (DECID ph -> STAB ph )
Theorem	stdcndc 840	A formula is decidable if and only if its negation is decidable and it is stable (that is, it is testable and stable). (Contributed by David A. Wheeler, 13-Aug-2018.) (Proof shortened by BJ, 28-Oct-2023.)
|- ( (STAB ph /\ DECID -. ph ) <-> DECID ph )
Theorem	stdcndcOLD 841	Obsolete version of stdcndc 840 as of 28-Oct-2023. (Contributed by David A. Wheeler, 13-Aug-2018.) (Proof modification is discouraged.) (New usage is discouraged.)
|- ( (STAB ph /\ DECID -. ph ) <-> DECID ph )
Theorem	stdcn 842	A formula is stable if and only if the decidability of its negation implies its decidability. Note that the right-hand side of this biconditional is the converse of dcn 837. (Contributed by BJ, 18-Nov-2023.)
|- (STAB ph <-> (DECID -. ph -> DECID ph ) )
Theorem	dcnn 843	Decidability of the negation of a proposition is equivalent to decidability of its double negation. See also dcn 837. The relation between dcn 837 and dcnn 843 is analogous to that between notnot 624 and notnotnot 629 (and directly stems from it). Using the notion of "testable proposition" (proposition whose negation is decidable), dcnn 843 means that a proposition is testable if and only if its negation is testable, and dcn 837 means that decidability implies testability. (Contributed by David A. Wheeler, 6-Dec-2018.) (Proof shortened by BJ, 25-Nov-2023.)
|- (DECID -. ph <-> DECID -. -. ph )
Theorem	dcnnOLD 844	Obsolete proof of dcnnOLD 844 as of 25-Nov-2023. (Contributed by David A. Wheeler, 6-Dec-2018.) (Proof modification is discouraged.) (New usage is discouraged.)
|- (DECID -. ph <-> DECID -. -. ph )
Theorem	nnexmid 845	Double negation of decidability of a formula. See also comment of nndc 846 to avoid a pitfall that could come from the label "nnexmid". This theorem can also be proved from bj-nnor 13769 as in bj-nndcALT 13793. (Contributed by BJ, 9-Oct-2019.)
|- -. -. ( ph \/ -. ph )
Theorem	nndc 846	Double negation of decidability of a formula. Intuitionistic logic refutes the negation of decidability (but does not prove decidability) of any formula. This should not trick the reader into thinking that -. -. EXMID is provable in intuitionistic logic. Indeed, if we could quantify over formula metavariables, then generalizing nnexmid 845 over ph would give " |- A. ph -. -. DECID ph", but EXMID is " A. phDECID ph", so proving -. -. EXMID would amount to proving " -. -. A. phDECID ph", which is not implied by the above theorem. Indeed, the converse of nnal 1642 does not hold. Since our system does not allow quantification over formula metavariables, we can reproduce this argument by representing formulas as subsets of ~P 1o, like we do in our definition of EXMID (df-exmid 4181): then, we can prove A. x e. ~P 1o -. -. DECID x = 1o but we cannot prove -. -. A. x e. ~P 1oDECID x = 1o because the converse of nnral 2460 does not hold. Actually, -. -. EXMID is not provable in intuitionistic logic since intuitionistic logic has models satisfying -. EXMID and noncontradiction holds (pm3.24 688). (Contributed by BJ, 9-Oct-2019.) Add explanation on non-provability of -. -. EXMID. (Revised by BJ, 11-Aug-2024.)
|- -. -. DECID ph
1.2.9  Theorems of decidable propositions Many theorems of logic hold in intuitionistic logic just as they do in classical (non-intuitionistic) logic, for all propositions. Other theorems only hold for decidable propositions, such as the law of the excluded middle (df-dc 830), double negation elimination (notnotrdc 838), or contraposition (condc 848). Our goal is to prove all well-known or important classical theorems, but with suitable decidability conditions so that the proofs follow from intuitionistic axioms. This section is focused on such proofs, given decidability conditions. Many theorems of this section actually hold for stable propositions (see df-stab 826). Decidable propositions are stable (dcstab 839), but the converse need not hold.
Theorem	const 847	Contraposition when the antecedent is a negated stable proposition. See comment of condc 848. (Contributed by BJ, 18-Nov-2023.) (Proof shortened by BJ, 11-Nov-2024.)
|- (STAB ph -> ( ( -. ph -> -. ps ) -> ( ps -> ph ) ) )
Theorem	condc 848	Contraposition of a decidable proposition. This theorem swaps or "transposes" the order of the consequents when negation is removed. An informal example is that the statement "if there are no clouds in the sky, it is not raining" implies the statement "if it is raining, there are clouds in the sky". This theorem (without the decidability condition, of course) is called Transp or "the principle of transposition" in Principia Mathematica (Theorem *2.17 of [WhiteheadRussell] p. 103) and is Axiom A3 of [Margaris] p. 49. We will also use the term "contraposition" for this principle, although the reader is advised that in the field of philosophical logic, "contraposition" has a different technical meaning. (Contributed by Jim Kingdon, 13-Mar-2018.) (Proof shortened by BJ, 18-Nov-2023.)
|- (DECID ph -> ( ( -. ph -> -. ps ) -> ( ps -> ph ) ) )
Theorem	condcOLD 849	Obsolete proof of condc 848 as of 18-Nov-2023. (Contributed by Jim Kingdon, 13-Mar-2018.) (Proof modification is discouraged.) (New usage is discouraged.)
|- (DECID ph -> ( ( -. ph -> -. ps ) -> ( ps -> ph ) ) )
Theorem	pm2.18dc 850	Proof by contradiction for a decidable proposition. Based on Theorem *2.18 of [WhiteheadRussell] p. 103 (also called Clavius law). Intuitionistically it requires a decidability assumption, but compare with pm2.01 611 which does not. (Contributed by Jim Kingdon, 24-Mar-2018.)
|- (DECID ph -> ( ( -. ph -> ph ) -> ph ) )
Theorem	con1dc 851	Contraposition for a decidable proposition. Based on theorem *2.15 of [WhiteheadRussell] p. 102. (Contributed by Jim Kingdon, 29-Mar-2018.)
|- (DECID ph -> ( ( -. ph -> ps ) -> ( -. ps -> ph ) ) )
Theorem	con4biddc 852	A contraposition deduction. (Contributed by Jim Kingdon, 18-May-2018.)
|- ( ph -> (DECID ps -> (DECID ch -> ( -. ps <-> -. ch ) ) ) )   =>    |- ( ph -> (DECID ps -> (DECID ch -> ( ps <-> ch ) ) ) )
Theorem	impidc 853	An importation inference for a decidable consequent. (Contributed by Jim Kingdon, 30-Apr-2018.)
|- (DECID ch -> ( ph -> ( ps -> ch ) ) )   =>    |- (DECID ch -> ( -. ( ph -> -. ps ) -> ch ) )
Theorem	simprimdc 854	Simplification given a decidable proposition. Similar to Theorem *3.27 (Simp) of [WhiteheadRussell] p. 112. (Contributed by Jim Kingdon, 30-Apr-2018.)
|- (DECID ps -> ( -. ( ph -> -. ps ) -> ps ) )
Theorem	simplimdc 855	Simplification for a decidable proposition. Similar to Theorem *3.26 (Simp) of [WhiteheadRussell] p. 112. (Contributed by Jim Kingdon, 29-Mar-2018.)
|- (DECID ph -> ( -. ( ph -> ps ) -> ph ) )
Theorem	pm2.61ddc 856	Deduction eliminating a decidable antecedent. (Contributed by Jim Kingdon, 4-May-2018.)
|- ( ph -> ( ps -> ch ) )   &    |- ( ph -> ( -. ps -> ch ) )   =>    |- (DECID ps -> ( ph -> ch ) )
Theorem	pm2.6dc 857	Case elimination for a decidable proposition. Based on theorem *2.6 of [WhiteheadRussell] p. 107. (Contributed by Jim Kingdon, 25-Mar-2018.)
|- (DECID ph -> ( ( -. ph -> ps ) -> ( ( ph -> ps ) -> ps ) ) )
Theorem	jadc 858	Inference forming an implication from the antecedents of two premises, where a decidable antecedent is negated. (Contributed by Jim Kingdon, 25-Mar-2018.)
|- (DECID ph -> ( -. ph -> ch ) )   &    |- ( ps -> ch )   =>    |- (DECID ph -> ( ( ph -> ps ) -> ch ) )
Theorem	jaddc 859	Deduction forming an implication from the antecedents of two premises, where a decidable antecedent is negated. (Contributed by Jim Kingdon, 26-Mar-2018.)
|- ( ph -> (DECID ps -> ( -. ps -> th ) ) )   &    |- ( ph -> ( ch -> th ) )   =>    |- ( ph -> (DECID ps -> ( ( ps -> ch ) -> th ) ) )
Theorem	pm2.61dc 860	Case elimination for a decidable proposition. Theorem *2.61 of [WhiteheadRussell] p. 107 under a decidability condition. (Contributed by Jim Kingdon, 29-Mar-2018.)
|- (DECID ph -> ( ( ph -> ps ) -> ( ( -. ph -> ps ) -> ps ) ) )
Theorem	pm2.5gdc 861	Negating an implication for a decidable antecedent. General instance of Theorem *2.5 of [WhiteheadRussell] p. 107 under a decidability condition. (Contributed by Jim Kingdon, 29-Mar-2018.)
|- (DECID ph -> ( -. ( ph -> ps ) -> ( -. ph -> ch ) ) )
Theorem	pm2.5dc 862	Negating an implication for a decidable antecedent. Theorem *2.5 of [WhiteheadRussell] p. 107 under a decidability condition. (Contributed by Jim Kingdon, 29-Mar-2018.)
|- (DECID ph -> ( -. ( ph -> ps ) -> ( -. ph -> ps ) ) )
Theorem	pm2.521gdc 863	A general instance of Theorem *2.521 of [WhiteheadRussell] p. 107, under a decidability condition. (Contributed by BJ, 28-Oct-2023.)
|- (DECID ph -> ( -. ( ph -> ps ) -> ( ch -> ph ) ) )
Theorem	pm2.521dc 864	Theorem *2.521 of [WhiteheadRussell] p. 107, but with an additional decidability condition. Note that by replacing in proof pm2.52 651 with conax1k 649, we obtain a proof of the more general instance where the last occurrence of ph is replaced with any ch. (Contributed by Jim Kingdon, 5-May-2018.)
|- (DECID ph -> ( -. ( ph -> ps ) -> ( ps -> ph ) ) )
Theorem	pm2.521dcALT 865	Alternate proof of pm2.521dc 864. (Contributed by Jim Kingdon, 5-May-2018.) (Proof modification is discouraged.) (New usage is discouraged.)
|- (DECID ph -> ( -. ( ph -> ps ) -> ( ps -> ph ) ) )
Theorem	con34bdc 866	Contraposition. Theorem *4.1 of [WhiteheadRussell] p. 116, but for a decidable proposition. (Contributed by Jim Kingdon, 24-Apr-2018.)
|- (DECID ps -> ( ( ph -> ps ) <-> ( -. ps -> -. ph ) ) )
Theorem	notnotbdc 867	Double negation equivalence for a decidable proposition. Like Theorem *4.13 of [WhiteheadRussell] p. 117, but with a decidability antecendent. The forward direction, notnot 624, holds for all propositions, not just decidable ones. (Contributed by Jim Kingdon, 13-Mar-2018.)
|- (DECID ph -> ( ph <-> -. -. ph ) )
Theorem	con1biimdc 868	Contraposition. (Contributed by Jim Kingdon, 4-Apr-2018.)
|- (DECID ph -> ( ( -. ph <-> ps ) -> ( -. ps <-> ph ) ) )
Theorem	con1bidc 869	Contraposition. (Contributed by Jim Kingdon, 17-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( ( -. ph <-> ps ) <-> ( -. ps <-> ph ) ) ) )
Theorem	con2bidc 870	Contraposition. (Contributed by Jim Kingdon, 17-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( ( ph <-> -. ps ) <-> ( ps <-> -. ph ) ) ) )
Theorem	con1biddc 871	A contraposition deduction. (Contributed by Jim Kingdon, 4-Apr-2018.)
|- ( ph -> (DECID ps -> ( -. ps <-> ch ) ) )   =>    |- ( ph -> (DECID ps -> ( -. ch <-> ps ) ) )
Theorem	con1biidc 872	A contraposition inference. (Contributed by Jim Kingdon, 15-Mar-2018.)
|- (DECID ph -> ( -. ph <-> ps ) )   =>    |- (DECID ph -> ( -. ps <-> ph ) )
Theorem	con1bdc 873	Contraposition. Bidirectional version of con1dc 851. (Contributed by NM, 5-Aug-1993.)
|- (DECID ph -> (DECID ps -> ( ( -. ph -> ps ) <-> ( -. ps -> ph ) ) ) )
Theorem	con2biidc 874	A contraposition inference. (Contributed by Jim Kingdon, 15-Mar-2018.)
|- (DECID ps -> ( ph <-> -. ps ) )   =>    |- (DECID ps -> ( ps <-> -. ph ) )
Theorem	con2biddc 875	A contraposition deduction. (Contributed by Jim Kingdon, 11-Apr-2018.)
|- ( ph -> (DECID ch -> ( ps <-> -. ch ) ) )   =>    |- ( ph -> (DECID ch -> ( ch <-> -. ps ) ) )
Theorem	condandc 876	Proof by contradiction. This only holds for decidable propositions, as it is part of the family of theorems which assume -. ps, derive a contradiction, and therefore conclude ps. By contrast, assuming ps, deriving a contradiction, and therefore concluding -. ps, as in pm2.65 654, is valid for all propositions. (Contributed by Jim Kingdon, 13-May-2018.)
|- ( ( ph /\ -. ps ) -> ch )   &    |- ( ( ph /\ -. ps ) -> -. ch )   =>    |- (DECID ps -> ( ph -> ps ) )
Theorem	bijadc 877	Combine antecedents into a single biconditional. This inference is reminiscent of jadc 858. (Contributed by Jim Kingdon, 4-May-2018.)
|- ( ph -> ( ps -> ch ) )   &    |- ( -. ph -> ( -. ps -> ch ) )   =>    |- (DECID ps -> ( ( ph <-> ps ) -> ch ) )
Theorem	pm5.18dc 878	Relationship between an equivalence and an equivalence with some negation, for decidable propositions. Based on theorem *5.18 of [WhiteheadRussell] p. 124. Given decidability, we can consider -. ( ph <-> -. ps ) to represent "negated exclusive-or". (Contributed by Jim Kingdon, 4-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( ( ph <-> ps ) <-> -. ( ph <-> -. ps ) ) ) )
Theorem	dfandc 879	Definition of 'and' in terms of negation and implication, for decidable propositions. The forward direction holds for all propositions, and can (basically) be found at pm3.2im 632. (Contributed by Jim Kingdon, 30-Apr-2018.)
|- (DECID ph -> (DECID ps -> ( ( ph /\ ps ) <-> -. ( ph -> -. ps ) ) ) )
Theorem	pm2.13dc 880	A decidable proposition or its triple negation is true. Theorem *2.13 of [WhiteheadRussell] p. 101 with decidability condition added. (Contributed by Jim Kingdon, 13-May-2018.)
|- (DECID ph -> ( ph \/ -. -. -. ph ) )
Theorem	pm4.63dc 881	Theorem *4.63 of [WhiteheadRussell] p. 120, for decidable propositions. (Contributed by Jim Kingdon, 1-May-2018.)
|- (DECID ph -> (DECID ps -> ( -. ( ph -> -. ps ) <-> ( ph /\ ps ) ) ) )
Theorem	pm4.67dc 882	Theorem *4.67 of [WhiteheadRussell] p. 120, for decidable propositions. (Contributed by Jim Kingdon, 1-May-2018.)
|- (DECID ph -> (DECID ps -> ( -. ( -. ph -> -. ps ) <-> ( -. ph /\ ps ) ) ) )
Theorem	imanst 883	Express implication in terms of conjunction. Theorem 3.4(27) of [Stoll] p. 176. (Contributed by NM, 12-Mar-1993.) (Proof shortened by Wolf Lammen, 30-Oct-2012.)
|- (STAB ps -> ( ( ph -> ps ) <-> -. ( ph /\ -. ps ) ) )
Theorem	imandc 884	Express implication in terms of conjunction. Theorem 3.4(27) of [Stoll] p. 176, with an added decidability condition. The forward direction, imanim 683, holds for all propositions, not just decidable ones. (Contributed by Jim Kingdon, 25-Apr-2018.)
|- (DECID ps -> ( ( ph -> ps ) <-> -. ( ph /\ -. ps ) ) )
Theorem	pm4.14dc 885	Theorem *4.14 of [WhiteheadRussell] p. 117, given a decidability condition. (Contributed by Jim Kingdon, 24-Apr-2018.)
|- (DECID ch -> ( ( ( ph /\ ps ) -> ch ) <-> ( ( ph /\ -. ch ) -> -. ps ) ) )
Theorem	pm2.54dc 886	Deriving disjunction from implication for a decidable proposition. Based on theorem *2.54 of [WhiteheadRussell] p. 107. The converse, pm2.53 717, holds whether the proposition is decidable or not. (Contributed by Jim Kingdon, 26-Mar-2018.)
|- (DECID ph -> ( ( -. ph -> ps ) -> ( ph \/ ps ) ) )
Theorem	dfordc 887	Definition of disjunction in terms of negation and implication for a decidable proposition. Based on definition of [Margaris] p. 49. One direction, pm2.53 717, holds for all propositions, not just decidable ones. (Contributed by Jim Kingdon, 26-Mar-2018.)
|- (DECID ph -> ( ( ph \/ ps ) <-> ( -. ph -> ps ) ) )
Theorem	pm2.25dc 888	Elimination of disjunction based on a disjunction, for a decidable proposition. Based on theorem *2.25 of [WhiteheadRussell] p. 104. (Contributed by NM, 3-Jan-2005.)
|- (DECID ph -> ( ph \/ ( ( ph \/ ps ) -> ps ) ) )
Theorem	pm2.68dc 889	Concluding disjunction from implication for a decidable proposition. Based on theorem *2.68 of [WhiteheadRussell] p. 108. Converse of pm2.62 743 and one half of dfor2dc 890. (Contributed by Jim Kingdon, 27-Mar-2018.)
|- (DECID ph -> ( ( ( ph -> ps ) -> ps ) -> ( ph \/ ps ) ) )
Theorem	dfor2dc 890	Disjunction expressed in terms of implication only, for a decidable proposition. Based on theorem *5.25 of [WhiteheadRussell] p. 124. (Contributed by Jim Kingdon, 27-Mar-2018.)
|- (DECID ph -> ( ( ph \/ ps ) <-> ( ( ph -> ps ) -> ps ) ) )
Theorem	imimorbdc 891	Simplify an implication between implications, for a decidable proposition. (Contributed by Jim Kingdon, 18-Mar-2018.)
|- (DECID ps -> ( ( ( ps -> ch ) -> ( ph -> ch ) ) <-> ( ph -> ( ps \/ ch ) ) ) )
Theorem	imordc 892	Implication in terms of disjunction for a decidable proposition. Based on theorem *4.6 of [WhiteheadRussell] p. 120. The reverse direction, imorr 716, holds for all propositions. (Contributed by Jim Kingdon, 20-Apr-2018.)
|- (DECID ph -> ( ( ph -> ps ) <-> ( -. ph \/ ps ) ) )
Theorem	pm4.62dc 893	Implication in terms of disjunction. Like Theorem *4.62 of [WhiteheadRussell] p. 120, but for a decidable antecedent. (Contributed by Jim Kingdon, 21-Apr-2018.)
|- (DECID ph -> ( ( ph -> -. ps ) <-> ( -. ph \/ -. ps ) ) )
Theorem	ianordc 894	Negated conjunction in terms of disjunction (DeMorgan's law). Theorem *4.51 of [WhiteheadRussell] p. 120, but where one proposition is decidable. The reverse direction, pm3.14 748, holds for all propositions, but the equivalence only holds where one proposition is decidable. (Contributed by Jim Kingdon, 21-Apr-2018.)
|- (DECID ph -> ( -. ( ph /\ ps ) <-> ( -. ph \/ -. ps ) ) )
Theorem	pm4.64dc 895	Theorem *4.64 of [WhiteheadRussell] p. 120, given a decidability condition. The reverse direction, pm2.53 717, holds for all propositions. (Contributed by Jim Kingdon, 2-May-2018.)
|- (DECID ph -> ( ( -. ph -> ps ) <-> ( ph \/ ps ) ) )
Theorem	pm4.66dc 896	Theorem *4.66 of [WhiteheadRussell] p. 120, given a decidability condition. (Contributed by Jim Kingdon, 2-May-2018.)
|- (DECID ph -> ( ( -. ph -> -. ps ) <-> ( ph \/ -. ps ) ) )
Theorem	pm4.54dc 897	Theorem *4.54 of [WhiteheadRussell] p. 120, for decidable propositions. One form of DeMorgan's law. (Contributed by Jim Kingdon, 2-May-2018.)
|- (DECID ph -> (DECID ps -> ( ( -. ph /\ ps ) <-> -. ( ph \/ -. ps ) ) ) )
Theorem	pm4.79dc 898	Equivalence between a disjunction of two implications, and a conjunction and an implication. Based on theorem *4.79 of [WhiteheadRussell] p. 121 but with additional decidability antecedents. (Contributed by Jim Kingdon, 28-Mar-2018.)
|- (DECID ph -> (DECID ps -> ( ( ( ps -> ph ) \/ ( ch -> ph ) ) <-> ( ( ps /\ ch ) -> ph ) ) ) )
Theorem	pm5.17dc 899	Two ways of stating exclusive-or which are equivalent for a decidable proposition. Based on theorem *5.17 of [WhiteheadRussell] p. 124. (Contributed by Jim Kingdon, 16-Apr-2018.)
|- (DECID ps -> ( ( ( ph \/ ps ) /\ -. ( ph /\ ps ) ) <-> ( ph <-> -. ps ) ) )
Theorem	pm2.85dc 900	Reverse distribution of disjunction over implication, given decidability. Based on theorem *2.85 of [WhiteheadRussell] p. 108. (Contributed by Jim Kingdon, 1-Apr-2018.)
|- (DECID ph -> ( ( ( ph \/ ps ) -> ( ph \/ ch ) ) -> ( ph \/ ( ps -> ch ) ) ) )