Jeffrey Shallit schrieb: >> Jeffrey Shallit schrieb: >>> -7 >>> >>>> Does there exist a non-square number which is square (mod 2^n) for every n >>>>> = 0? >>> >> Mod[-7, 2^6] is 57 > > Yes, and 57 is a square mod 2^6. 11^2 == 57 == -7 (mod 64). > > > It seems this works not only for -7 but for -A004215(k) (at least for 1 <= n <= 500)