A175915 Odd k such that sigma(k) - 2*k (the abundance of k) is a square >= 0.

2205, 312896925, 1874196225, 3356107425, 4181881725, 4987853325, 6946213725, 8860349025, 13250605725, 17989980525, 25443641025, 32177090925, 38286871425, 55531730925, 75756402225, 113679103725, 152738872209, 170812073025, 179835853725, 186091427025

Offset

1,1

Comments

sqrt(sigma(a(n))-2*a(n)): 6, 3432, 8250, 11022, 19376.

Links

Max Alekseyev, Table of n, a(n) for n = 1..38

Index entries for sequences where odd perfect numbers must occur, if they exist at all.

Example

a(1) = 2205 = A109510(29).

Prog

(PARI) is(n)=n%2&&issquare(sigma(n)-2*n); \\ Charles R Greathouse IV, Dec 06 2011

Crossrefs

Odd terms of A109510.

Contains as subsequences the odd terms (if any) of: A088832, A141547, A275996.

Cf. A063788, A083874, A088012, A292558

Sequence in context: A252397 A252392 A387709 * A098192 A004931 A004951

Adjacent sequences: A175912 A175913 A175914 * A175916 A175917 A175918

Keyword

nonn

Author

Zak Seidov, Oct 15 2010

Extensions

a(2)-a(5) from Max Alekseyev, Oct 20 2010

a(6)-a(18) from Donovan Johnson, Nov 10 2010

a(19)-a(20) from Donovan Johnson, Dec 06 2011

Name edited by Antti Karttunen, Sep 18 2025

Status

approved