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A071864 Nonprime n such that the number of elements in the continued fraction for Sum_{d|n} 1/d equals tau(n), the number of divisors of n. 2
1, 4, 9, 14, 15, 21, 25, 49, 51, 55, 57, 63, 95, 98, 99, 115, 116, 121, 147, 161, 169, 172, 175, 188, 195, 203, 236, 244, 245, 247, 265, 284, 287, 289, 297, 299, 322, 328, 329, 351, 356, 361, 363, 370, 371, 374, 387, 406, 412, 413, 418, 423, 425, 437, 465, 488 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If p is prime p^2 is in the sequence since the continued fraction for Sum_{d|n} 1/d is [1, p-1, p+1] and there are 3 divisors for p^2.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

MATHEMATICA

aQ[n_] := ! PrimeQ[n] && Length@ContinuedFraction[DivisorSigma[1, n]/n] ==  DivisorSigma[0, n]; Select[Range[488], aQ] (* Amiram Eldar, Aug 30 2019 *)

PROG

(PARI) for(n=1, 1000, if(length(contfrac(sumdiv(n, d, 1/d)))==numdiv(n)*(1-isprime(n)), print1(n, ", ")))

CROSSREFS

Cf. A000005, A017665, A017666, A071862.

Sequence in context: A244037 A091880 A141560 * A312987 A050986 A284956

Adjacent sequences:  A071861 A071862 A071863 * A071865 A071866 A071867

KEYWORD

easy,nonn

AUTHOR

Benoit Cloitre, Jun 09 2002

STATUS

approved

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Last modified September 30 22:26 EDT 2020. Contains 337440 sequences. (Running on oeis4.)