A232538 - OEIS

%I #16 Sep 12 2019 08:47:29

%S 33,136,145,261,897,1441,2016,2241,2353,3808,4320,7201,17101,26937,

%T 30721,32896,46593,70561,148960,151633,169345,174592,208801,400401,

%U 578593,712801,803800,1040401,1103233,1596673,2265121,2377089,3330001,4357153,5953024,5962321

%N Numbers n such that (n(n+1)/2) modulo sigma(n) = n.

%C Also numbers n such that antisigma(n) modulo sigma(n) = n. Antisigma(n) = A024816(n) = the sum of the nondivisors of n that are between 1 and n, sigma(n) = A000203(n) = the sum of the divisors of n.

%C Numbers n such that A232324(n) = n.

%C a(19)  > 10^5.

%F A232324(a(n)) = n.

%e 136 is in sequence because antisigma(136) mod sigma(136) = 9046 mod 270 = 136.

%t Select[Range[6*10^6],Mod[(#(#+1))/2,DivisorSigma[1,#]]==#&] (* _Harvey P. Dale_, Sep 12 2019 *)

%o (PARI) isok(n) = (n*(n+1)/2 - sigma(n)) % sigma(n) == n; \\ _Michel Marcus_, Nov 25 2013

%Y Cf. A000203, A076617, A024816, A232324, A232540.

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Nov 25 2013

%E More terms from _Michel Marcus_, Nov 25 2013