A231546 - OEIS

%I #20 May 28 2024 15:01:57

%S 15,207,958,1335,1365,1635,2686,2975,4365,14842,18874,19359,20146,

%T 24958,33999,36567,42819,56565,64666,74919,79827,79834,84135,92686,

%U 109215,111507,116938,122074,138238,147455,161002,162603,166935,174718,190774,193894,201598

%N Numbers k such that sigma(k) = sigma(k-1).

%C Also, numbers k such that k = antisigma(k) - antisigma(k-1), where antisigma(k) = A024816(k) = the sum of the non-divisors of k that are between 1 and k.

%H Amiram Eldar, <a href="/A231546/b231546.txt">Table of n, a(n) for n = 1..10135</a> (calculated using the b-file at A002961; terms 1..550 from Hugo Pfoertner)

%F a(n) = A002961(n) + 1.

%e 15 = antisigma(15) - antisigma(14) = 96 - 81.

%t SequencePosition[DivisorSigma[1,Range[210000]],{x_,x_}][[;;,2]] (* _Harvey P. Dale_, May 28 2024 *)

%o (PARI) n=0;sp=sigma(2);for(k=3,oo,my(s=sigma(k));if(s==sp,print1(k,", ");n++;if(n>36,break));sp=s) \\ _Hugo Pfoertner_, Mar 06 2020

%Y Cf. A002961, A024816 (antisigma(n)), A231545 (numbers n such that antisigma(n) = antisigma(n-1)).

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Nov 11 2013

%E More terms from _Hugo Pfoertner_, Mar 06 2020