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A318059 a(n) is the numerator of sigma(sigma(n))/n. 5
1, 2, 7, 2, 12, 14, 15, 3, 14, 39, 28, 14, 24, 30, 4, 2, 39, 28, 42, 24, 3, 91, 60, 7, 32, 48, 10, 30, 72, 13, 63, 13, 124, 60, 124, 28, 60, 84, 40, 117, 96, 6, 84, 56, 56, 195, 124, 14, 80, 64, 65, 171, 120, 20, 39, 45, 62, 117, 168, 8, 96, 126, 10, 2, 224, 403, 126, 78, 84, 403 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
LINKS
FORMULA
For n=2, sigma(sigma(n)) = 4, so a(2) = 2.
EXAMPLE
Fractions begin with 1, 2, 7/3, 2, 12/5, 14/3, 15/7, 3, 14/9, 39/10, 28/11, 14/3, ...
MAPLE
seq(numer((numtheory:-sigma@@2)(n)/n), n=1..100); # Robert Israel, Aug 14 2018
MATHEMATICA
a[n_] := Numerator[DivisorSigma[1, DivisorSigma[1, n]]/n]; Array[a, 100] (* Amiram Eldar, Apr 04 2024 *)
PROG
(PARI) a(n) = numerator(sigma(sigma(n))/n);
CROSSREFS
Cf. A000203 (sigma), A051027.
Cf. A098223 (when denominator is 1), A318060 (denominator).
Sequence in context: A223149 A222853 A222654 * A212835 A089123 A281751
KEYWORD
nonn,frac
AUTHOR
Michel Marcus, Aug 14 2018
STATUS
approved

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Last modified May 29 13:50 EDT 2024. Contains 372949 sequences. (Running on oeis4.)