Metamath Proof Explorer


Theorem mnfltpnf

Description: Minus infinity is less than plus infinity. (Contributed by NM, 14-Oct-2005)

Ref Expression
Assertion mnfltpnf -∞ < +∞

Proof

Step Hyp Ref Expression
1 eqid -∞ = -∞
2 eqid +∞ = +∞
3 olc ( ( -∞ = -∞ ∧ +∞ = +∞ ) → ( ( ( -∞ ∈ ℝ ∧ +∞ ∈ ℝ ) ∧ -∞ < +∞ ) ∨ ( -∞ = -∞ ∧ +∞ = +∞ ) ) )
4 1 2 3 mp2an ( ( ( -∞ ∈ ℝ ∧ +∞ ∈ ℝ ) ∧ -∞ < +∞ ) ∨ ( -∞ = -∞ ∧ +∞ = +∞ ) )
5 4 orci ( ( ( ( -∞ ∈ ℝ ∧ +∞ ∈ ℝ ) ∧ -∞ < +∞ ) ∨ ( -∞ = -∞ ∧ +∞ = +∞ ) ) ∨ ( ( -∞ ∈ ℝ ∧ +∞ = +∞ ) ∨ ( -∞ = -∞ ∧ +∞ ∈ ℝ ) ) )
6 mnfxr -∞ ∈ ℝ*
7 pnfxr +∞ ∈ ℝ*
8 ltxr ( ( -∞ ∈ ℝ* ∧ +∞ ∈ ℝ* ) → ( -∞ < +∞ ↔ ( ( ( ( -∞ ∈ ℝ ∧ +∞ ∈ ℝ ) ∧ -∞ < +∞ ) ∨ ( -∞ = -∞ ∧ +∞ = +∞ ) ) ∨ ( ( -∞ ∈ ℝ ∧ +∞ = +∞ ) ∨ ( -∞ = -∞ ∧ +∞ ∈ ℝ ) ) ) ) )
9 6 7 8 mp2an ( -∞ < +∞ ↔ ( ( ( ( -∞ ∈ ℝ ∧ +∞ ∈ ℝ ) ∧ -∞ < +∞ ) ∨ ( -∞ = -∞ ∧ +∞ = +∞ ) ) ∨ ( ( -∞ ∈ ℝ ∧ +∞ = +∞ ) ∨ ( -∞ = -∞ ∧ +∞ ∈ ℝ ) ) ) )
10 5 9 mpbir -∞ < +∞