### isabelle

#### Is there a lemma like “∃x. a^x = b” proved in Isabelle?

Does anyone know where a lemma similar to ∃(x::real). a^x = (b::real) might be found? I couldn't find something like that in the 'query', but it seems pretty handy.

You need a few more assumptions on a and b and you need to use the powr operator instead of ^, since ^ is only for the n-th power where n is a natural number. powr on the other hand is for any non-negative real number raised to the power of any other real number. (or similarly for complex numbers) lemma fixes a b :: real assumes "a > 0" "a ≠ 1" "b > 0" shows "∃x. a powr x = b" proof from assms show "a powr (log a b) = b" by simp qed

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