François G. Dorais
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 Mar 4 awarded Enlightened Mar 4 awarded Nice Answer Feb 4 comment Solving the recurrence $A_n = \sum_{k=1}^{n} 2^{k+1} A_{n-k}$ I wish Martin had asked this a few days earlier. I just assigned some similar problems to my linear algebra students and this one actually is much nicer than the artificial ones I came up with. Feb 4 revised Solving the recurrence $A_n = \sum_{k=1}^{n} 2^{k+1} A_{n-k}$ added 15 characters in body Feb 4 answered Solving the recurrence $A_n = \sum_{k=1}^{n} 2^{k+1} A_{n-k}$ Jan 31 awarded Nice Answer Jan 11 awarded Guru Jan 10 comment What's an example of an infinitesimal? @CarlMummert please see this MO question to understand user72694's comment : mathoverflow.net/questions/227945/… Dec 16 awarded Nice Answer Jul 20 awarded Yearling May 3 awarded Good Answer Feb 11 awarded Necromancer Dec 19 awarded Constituent Dec 10 awarded Caucus Oct 5 awarded Nice Answer Jul 20 awarded Yearling Jul 7 awarded Nice Answer Mar 8 comment Are there statements that are undecidable but not provably undecidable @JDH's objection is correct but there is more. The implicit assumption is actually that ZFC+Con(ZFC) is $\Sigma^0_1$-sound: that this theory does not prove any false $\Sigma^0_1$-statement. If ZFC+Con(ZFC) is not $\Sigma^0_1$-sound then there is a Turing machine $M$ such that ZFC + Con(ZFC) proves that "$M$ halts" but $M$ doesn't actually halt... Dec 23 comment Problems with introducing ordered pairs axiomatically Andrej, this doesn't answer the question as posed. The OP is asking about a global pairing function, not about the existence of products, which only gives local pairing functions. Dec 23 answered Problems with introducing ordered pairs axiomatically