This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A270698 #33 Nov 06 2023 00:26:40 %S A270698 561,1105,1729,1905,2465,3277,4033,4681,6601,8321,8481,10585,12801, %T A270698 15841,16705,18705,25761,29341,30121,33153,34945,41041,46657,49141, %U A270698 52633,62745,65281,74665,75361,80581,85489,87249,88357,104653,113201,115921,126217,129921 %N A270698 Composite numbers k == 1 (mod 4) such that (1 + i)^k == 1 + i (mod k), where i = sqrt(-1). %C A270698 From _Jianing Song_, Sep 05 2018: (Start) %C A270698 Numbers in A047713 that are congruent to 1 mod 4. Most terms are congruent to 1 mod 8. For terms congruent to 5 mod 8, see A244626. %C A270698 Also composite k == 1 (mod 4) such that (-4)^((k-1)/4) == 1 (mod k). Note that this is satisfied by all primes == 1 (mod 4), see A318898. (End) %H A270698 Amiram Eldar, <a href="/A270698/b270698.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..889 from Jianing Song using data from A047713) %t A270698 Select[1 + 4*Range[100000], PrimeQ[#] == False && PowerMod[1 + I, #, #] == 1 + I &] %o A270698 (PARI) forstep(n=5, 10^5, 4, if(Mod(2, n)^((n-1)/2)==kronecker(2, n) && !isprime(n), print1(n, ", "))) \\ _Jianing Song_, Sep 06 2018 %Y A270698 Subsequence of A001567 and A047713. %Y A270698 A244626 is a proper subsequence. %Y A270698 Cf. A006971, A270697, A318898. %K A270698 nonn %O A270698 1,1 %A A270698 _José María Grau Ribas_, Mar 21 2016