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A283744
Nonprime numbers k such that k divides sigma(k^2) - 1.
1
1, 6, 18, 20, 30, 80, 180, 225, 1968, 5494, 8280, 20820, 37976, 175848, 2635928, 3747827, 15737100, 1162783652, 1453479565, 27961574707, 155176161190
OFFSET
1,2
COMMENTS
Odd terms: 1, 225, 3747827, 1453479565, 27961574707,...
6 is the only semiprime in the sequence. - Robert Israel, Mar 16 2017
EXAMPLE
6 is in this sequence because 6 divides sigma(6^2) - 1 = 90.
MATHEMATICA
Select[Range[16000000], !PrimeQ[#] && Divisible[DivisorSigma[1, #^2] - 1, #] &] (* Indranil Ghosh, Mar 15 2017 *)
PROG
(Magma) [n: n in [1..2000] | not IsPrime(n) and Denominator((n^2)-1)/n) eq 1];
(PARI) for(n=1, 15737100, if(!isprime(n) && (sigma(n^2) - 1)%n==0, print1(n, ", "))) \\ Indranil Ghosh, Mar 15 2017
CROSSREFS
Subsequence of A283599.
Sequence in context: A114452 A376881 A253914 * A108762 A088724 A124276
KEYWORD
nonn,more
AUTHOR
EXTENSIONS
a(18)-a(21) from Giovanni Resta, Mar 16 2017
STATUS
approved