A068207 Numbers k such that the number of divisors of k^2-1 equals the number of divisors of k.

2, 6, 40, 54, 66, 70, 78, 80, 88, 110, 114, 128, 130, 182, 216, 222, 238, 250, 258, 276, 310, 318, 354, 366, 384, 402, 410, 418, 438, 456, 488, 498, 552, 594, 598, 616, 624, 632, 654, 678, 680, 682, 696, 726, 760, 762, 774, 786, 808, 830, 870, 906, 924, 928

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Formula

k such that A000005(k^2-1) = A000005(k).

Mathematica

Select[Range[1000], DivisorSigma[0, #] == DivisorSigma[0, #^2-1] &] (* Amiram Eldar, Jul 17 2019 *)

Crossrefs

Cf. A000005.

Sequence in context: A028300 A132192 A340299 * A331702 A288491 A212883

Adjacent sequences: A068204 A068205 A068206 * A068208 A068209 A068210

Keyword

easy,nonn

Author

Benoit Cloitre, Mar 23 2002

Status

approved