A187712 - OEIS

%I #17 Mar 22 2024 06:48:29

%S 10,20,33,40,76,136,145,207,261,385,464,528,588,897,931,1441,1519,

%T 1611,1816,1989,2016,2205,2241,2353,3280,3504,3724,3808,4067,4320,

%U 4864,5696,6256,7201,7345,8036,10688,10936,11376,13000,16840,17101,18625,19359,19504,19840

%N Composite numbers k such that k = (product of divisors of k) mod (sum of divisors of k).

%H Amiram Eldar, <a href="/A187712/b187712.txt">Table of n, a(n) for n = 1..1000</a>

%F A187711 INTERSECT A002808.

%t Select[Range[20000], CompositeQ[#] && PowerMod[#, DivisorSigma[0, #]/2, DivisorSigma[1, #]] == # &] (* _Amiram Eldar_, Mar 22 2024 *)

%o (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 is(n) = n > 1 && !isprime(n) && is1(n); \\ _Amiram Eldar_, Mar 22 2024

%Y Cf. A000203, A002808, A007955, A187680, A187711.

%K nonn

%O 1,1

%A _Juri-Stepan Gerasimov_, Mar 17 2011

%E More terms from _Amiram Eldar_, Mar 22 2024