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 A068992 Composite numbers k such that Sum_{d|k} sigma(d)/tau(d) is an integer. 1
 10, 15, 21, 26, 30, 33, 34, 35, 39, 49, 51, 55, 57, 58, 60, 65, 69, 70, 74, 75, 77, 78, 82, 85, 87, 91, 93, 95, 98, 102, 105, 106, 110, 111, 115, 119, 120, 122, 123, 129, 130, 133, 141, 143, 145, 146, 147, 155, 156, 159, 161, 165, 169, 170, 174, 177, 178, 182, 183 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS For p prime, contains p^2 if and only if p == 1 (mod 6). - Robert Israel, May 14 2019 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 MAPLE N:= 1000: V:= Vector(N): for d from 1 to N do r:= numtheory:-sigma(d)/numtheory:-tau(d); C:= [seq(i, i=d..N, d)]; V[C]:= V[C] +~ r od: select(t -> not(isprime(t)) and V[t]::integer, [\$2..N]); # Robert Israel, May 14 2019 MATHEMATICA q[n_] := CompositeQ[n] && IntegerQ @ DivisorSum[n, Divide @@ DivisorSigma[{1, 0}, #] &]; Select[Range[200], q] (* Amiram Eldar, Jun 08 2022 *) CROSSREFS Contains A024556. Cf. A000005, A000203. Sequence in context: A108614 A336548 A115708 * A325901 A098564 A168103 Adjacent sequences: A068989 A068990 A068991 * A068993 A068994 A068995 KEYWORD easy,nonn AUTHOR Benoit Cloitre, Apr 06 2002 STATUS approved

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Last modified March 24 04:28 EDT 2023. Contains 361454 sequences. (Running on oeis4.)