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
KEYWORD
nonn
AUTHOR
Alex Ratushnyak, Jul 03 2013
STATUS
approved