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 A187712 #17 Mar 22 2024 06:48:29 %S A187712 10,20,33,40,76,136,145,207,261,385,464,528,588,897,931,1441,1519, %T A187712 1611,1816,1989,2016,2205,2241,2353,3280,3504,3724,3808,4067,4320, %U A187712 4864,5696,6256,7201,7345,8036,10688,10936,11376,13000,16840,17101,18625,19359,19504,19840 %N A187712 Composite numbers k such that k = (product of divisors of k) mod (sum of divisors of k). %H A187712 Amiram Eldar, <a href="/A187712/b187712.txt">Table of n, a(n) for n = 1..1000</a> %F A187712 A187711 INTERSECT A002808. %t A187712 Select[Range[20000], CompositeQ[#] && PowerMod[#, DivisorSigma[0, #]/2, DivisorSigma[1, #]] == # &] (* _Amiram Eldar_, Mar 22 2024 *) %o A187712 (PARI) is1(n) = my(f = factor(n), s = sigma(f), d = numdiv(f)); if(d%2, Mod(sqrtint(n), s)^d, Mod(n, s)^(d/2)) == n; %o A187712 is(n) = n > 1 && !isprime(n) && is1(n); \\ _Amiram Eldar_, Mar 22 2024 %Y A187712 Cf. A000203, A002808, A007955, A187680, A187711. %K A187712 nonn %O A187712 1,1 %A A187712 _Juri-Stepan Gerasimov_, Mar 17 2011 %E A187712 More terms from _Amiram Eldar_, Mar 22 2024