A231546 Numbers k such that sigma(k) = sigma(k-1).

15, 207, 958, 1335, 1365, 1635, 2686, 2975, 4365, 14842, 18874, 19359, 20146, 24958, 33999, 36567, 42819, 56565, 64666, 74919, 79827, 79834, 84135, 92686, 109215, 111507, 116938, 122074, 138238, 147455, 161002, 162603, 166935, 174718, 190774, 193894, 201598

Offset: 1

Author

Jaroslav Krizek, Nov 11 2013

Keywords

• nonn

Comments

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.

Examples

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

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10135 (calculated using the b-file at A002961; terms 1..550 from Hugo Pfoertner)

Crossrefs

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

Programs

Mathematica

SequencePosition[DivisorSigma[1,Range[210000]],{x_,x_}][[;;,2]] (* Harvey P. Dale, May 28 2024 *)

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

Formula

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

Extensions

More terms from Hugo Pfoertner, Mar 06 2020