A227904 - OEIS

%I #21 Jan 10 2022 22:24:15

%S 1,2,39,78,100,126,434,501,1313,54111,359466,523219,6601441,8034674,

%T 54092207,64149290,158882288,3016740661,20951813922,52815759197,

%U 120508871819

%N Numbers k such that Sum_{j=1..k} antisigma(j) == 0 (mod sigma(k)).

%C Tested up to k = 10^6.

%C a(22) > 2.1774*10^11. - _Kevin P. Thompson_, Jan 10 2022

%e Sum_{j=1..39} antisigma(j) = 9408, sigma(39) = 56 and 9408 mod 56 = 0, so 39 is a term.

%p with(numtheory); P:=proc(q) local a, n; a:=0;

%p for n from 3 to q do a:=a+n*(n+1)/2-sigma(n);

%p if (a mod sigma(n))=0 then print(n); fi; od; end: P(10^9);

%Y Cf. A000203, A024816, A076664, A168127.

%K nonn,more

%O 1,2

%A _Paolo P. Lava_, Oct 15 2013

%E a(13)-a(17) from _Donovan Johnson_, Oct 15 2013

%E a(18)-a(21) from _Kevin P. Thompson_, Jan 10 2022