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 A188540 Numbers k such that d(k+2) = 2*d(k) where d(k) is the number of divisors of k (A000005). 3
 1, 13, 19, 22, 31, 37, 38, 53, 67, 83, 86, 89, 109, 113, 124, 127, 131, 133, 134, 139, 148, 157, 169, 181, 187, 199, 211, 233, 251, 253, 257, 263, 292, 293, 295, 307, 310, 317, 326, 328, 337, 338, 343, 353, 355, 361, 376, 379, 389, 401, 406, 409, 412, 422, 427, 438, 443, 449, 453 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE 1 is a term since d(1+2) = d(3) = 2 = 2*d(1). MATHEMATICA Select[Range[500], DivisorSigma[0, # + 2] == 2 * DivisorSigma[0, #] &] (* Amiram Eldar, Jan 17 2021 *) PROG (PARI) isok(k) = numdiv(k+2) == 2*numdiv(k); \\ Michel Marcus, Jan 17 2021 CROSSREFS Cf. A000005, A063446, A160921. Sequence in context: A118845 A050717 A335034 * A088184 A274407 A054986 Adjacent sequences:  A188537 A188538 A188539 * A188541 A188542 A188543 KEYWORD nonn,easy AUTHOR Juri-Stepan Gerasimov, Apr 03 2011 STATUS approved

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Last modified May 13 00:11 EDT 2021. Contains 343829 sequences. (Running on oeis4.)