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Line 2: EFA is a very weak logical system, whose [[proof theoretic ordinal]] is ω<sup>3</sup>, but still seems able to prove much of ordinary mathematics that can be stated in the language of first-order arithmetic. There are weak fragments of second-order arithmetic called RCA{{su\|p=\|b=0}} and WKL{{su\|p=\|b=0}} that have the same consistency strength as EFA and are conservative over it for Π{{su\|p=0\|b=2}} sentences, which are sometimes studied in [[reverse mathematics]]. ==Friedman's grand conjecture==

Elementary function arithmetic: Difference between revisions