A069826 Numbers k such that sigma(k^2-k-1) = k*(k+1).

8, 15, 24, 35, 80, 99, 120, 255, 399, 440, 960, 1520, 2024, 2115, 2915, 3024, 3135, 3599, 4224, 4355, 7395, 8835, 10200, 13224, 15875, 17160, 19880, 21024, 22200, 23715, 24024, 25280, 25599, 26895, 32760, 38415, 47960, 48399, 53360, 58080

Offset

1,1

Comments

Also numbers n of form k*(k+2) such that both n-(k+1) and n+(k+1) are prime (see A116214). - Klaus Brockhaus, Apr 17 2007

In other words, numbers n such that n+sqrt(n+1) and n-sqrt(n+1) are both prime. - César Aguilera, Jul 01 2013

Links

Amiram Eldar, Table of n, a(n) for n = 1..400

Mathematica

Select[Range[60000], DivisorSigma[1, #^2-#-1]==#(#+1)&] (* Harvey P. Dale, Sep 13 2011 *)

Crossrefs

Cf. A000203, A005563, A116214.

Sequence in context: A015727 A034104 A034114 * A129076 A292949 A072177

Adjacent sequences: A069823 A069824 A069825 * A069827 A069828 A069829

Keyword

easy,nonn

Author

Benoit Cloitre, Apr 28 2002

Status

approved