cp's OEIS Frontend

A227231 Numbers k such that antisigma(k) mod k = k - 1.

%I A227231 #31 Jul 28 2025 18:42:21
%S A227231 1,3,4,5,7,11,13,17,19,23,29,31,36,37,41,43,47,53,59,61,67,71,73,79,
%T A227231 83,89,97,101,103,107,109,113,127,131,137,139,149,151,157,163,167,173,
%U A227231 179,181,191,193,197,199,211,223,227,229,233,239,241,251,257,263
%N A227231 Numbers k such that antisigma(k) mod k = k - 1.
%C A227231 Antisigma(k) = A024816(k) = sum of numbers less than k which do not divide k.
%C A227231 Union of A065091 (odd primes) and sequence nonprimes 1, 4, 36, ... (all terms < 10^5).
%C A227231 No more composite terms to 10^10. - _Charles R Greathouse IV_, Nov 02 2014
%H A227231 Jaroslav Krizek, <a href="/A227231/b227231.txt">Table of n, a(n) for n = 1..5000</a>
%e A227231 antisigma(36) mod 36 => 575 mod 36 = 35.
%o A227231 (Magma) [n: n in [1..1000] | n-1 eq ((n*(n+1) div 2-SumOfDivisors(n)) mod n)]; // _Jaroslav Krizek_, May 28 2014
%o A227231 (PARI) is(n)=(n*(n+1)/2-sigma(n)+1)%n==0 \\ _Charles R Greathouse IV_, Nov 02 2014
%Y A227231 Cf. A024816 (antisigma(n)), A065091, A229110 (antisigma(n) mod n).
%K A227231 nonn
%O A227231 1,2
%A A227231 _Jaroslav Krizek_, Sep 26 2013