Theorem xnegpnf 9828

Description: Minus +∞. Remark of [BourbakiTop1] p. IV.15. (Contributed by FL, 26-Dec-2011.)

Assertion

Ref	Expression
xnegpnf	⊢ -𝑒+∞ = -∞

Proof of Theorem xnegpnf

Step	Hyp	Ref	Expression
1		df-xneg 9772	. 2 ⊢ -𝑒+∞ = if(+∞ = +∞, -∞, if(+∞ = -∞, +∞, -+∞))
2		eqid 2177	. . 3 ⊢ +∞ = +∞
3	2	iftruei 3541	. 2 ⊢ if(+∞ = +∞, -∞, if(+∞ = -∞, +∞, -+∞)) = -∞
4	1, 3	eqtri 2198	1 ⊢ -𝑒+∞ = -∞

Syntax hints:   = wceq 1353  ifcif 3535  +∞cpnf 7989  -∞cmnf 7990  -cneg 8129  -_𝑒cxne 9769

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-ia1 106  ax-ia2 107  ax-ia3 108  ax-in2 615  ax-io 709  ax-5 1447  ax-7 1448  ax-gen 1449  ax-ie1 1493  ax-ie2 1494  ax-8 1504  ax-11 1506  ax-4 1510  ax-17 1526  ax-i9 1530  ax-ial 1534  ax-i5r 1535  ax-ext 2159

This theorem depends on definitions:  df-bi 117  df-nf 1461  df-sb 1763  df-clab 2164  df-cleq 2170  df-clel 2173  df-if 3536  df-xneg 9772

This theorem is referenced by:  xnegcl  9832  xnegneg  9833  xltnegi  9835  xnegid  9859  xnegdi  9868  xaddass2  9870  xsubge0  9881  xposdif  9882  xlesubadd  9883  xblss2ps  13907  xblss2  13908