A227240 Numbers n such that sigma(n) divides sigma(2*n) and sigma(2*n + 1).

1, 3, 5, 7, 11, 23, 29, 41, 53, 77, 83, 89, 103, 113, 131, 143, 173, 179, 191, 233, 239, 251, 281, 293, 359, 419, 431, 443, 491, 509, 533, 593, 641, 653, 659, 667, 683, 719, 743, 761, 807, 809, 817, 911, 953, 1013, 1019, 1031, 1049, 1073, 1103, 1223, 1229, 1289, 1409

Offset

1,2

Comments

Numbers such that 2*n and/or 2*n + 1 is also in the sequence: 1, 3, 5, 11, 41, 89, 179, 359, 509, 719, 743, ... (Cf. A007700).

Links

G. C. Greubel, Table of n, a(n) for n = 1..5000

Mathematica

Select[Range[1000], IntegerQ[DivisorSigma[1, 2#]/DivisorSigma[1, #]] && IntegerQ[DivisorSigma[1, 2# + 1]/DivisorSigma[1, #]] &] (* Alonso del Arte, Jul 15 2013 *)
Select[Range[1500], And@@Divisible[{DivisorSigma[1, 2#], DivisorSigma[1, 2#+1]}, DivisorSigma[1, #]]&] (* Harvey P. Dale, Feb 25 2016 *)

Prog

(PARI) isok(n) = my(sn=sigma(n)); !(sigma(2*n) % sn) && !(sigma(2*n+1) % sn); \\ Michel Marcus, Oct 02 2017

Crossrefs

Cf. A000203, A058072, A058073.

Sequence in context: A130759 A325155 A038890 * A360567 A226017 A303705

Adjacent sequences: A227237 A227238 A227239 * A227241 A227242 A227243

Keyword

nonn

Author

Alex Ratushnyak, Jul 03 2013

Status

approved