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A216781 Numbers such that numerator(sigma(n)/n) is odd and denominator(sigma(n)/n) is even. 3
2, 4, 8, 16, 18, 20, 24, 32, 36, 40, 48, 50, 52, 64, 68, 72, 80, 88, 96, 98, 100, 104, 112, 116, 128, 136, 144, 148, 152, 160, 162, 164, 176, 180, 192, 196, 200, 208, 212, 224, 232, 240, 242, 244, 256, 272, 288, 292, 296, 304, 320, 324, 328, 338, 344, 352 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(n) contains powers of 2 (A000079 except 1), and hemiperfect numbers (A055153, A141645, A159271, A160678).

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

EXAMPLE

sigma(2)/2 = 3/2 (odd/even).

MATHEMATICA

Select[Range[1000], OddQ[Numerator[DivisorSigma[1, #]/#]] && EvenQ[Denominator[DivisorSigma[1, #]/#]] &] (* Vincenzo Librandi, Jun 24 2014 *)

PROG

(PARI) oeab(n) = {for (i=1, n, ab = sigma(i)/i; if ((numerator(ab) % 2 == 1) && (denominator(ab) % 2 == 0), print1(i, ", ")); ); }

CROSSREFS

Cf. A216780, A216782, A000079, A055153, A141645, A159271, A160678.

Sequence in context: A048718 A018510 A018366 * A127066 A331382 A154362

Adjacent sequences:  A216778 A216779 A216780 * A216782 A216783 A216784

KEYWORD

nonn

AUTHOR

Michel Marcus, Sep 16 2012

STATUS

approved

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Last modified May 9 06:30 EDT 2021. Contains 343692 sequences. (Running on oeis4.)