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A246857
Numbers k such that sigma(k + sigma(k)) = 2*sigma(k).
3
2, 3, 5, 11, 23, 29, 41, 53, 83, 89, 113, 131, 173, 179, 191, 233, 239, 251, 281, 293, 329, 359, 413, 419, 431, 443, 491, 509, 593, 623, 641, 653, 659, 683, 719, 743, 761, 809, 869, 911, 953, 979, 1013, 1019, 1031, 1049, 1103, 1223, 1229, 1289, 1409, 1439, 1451
OFFSET
1,1
COMMENTS
Union of A005384 (Sophie Germain primes) and A246858.
First composite number in sequence is 329 (see A246858).
EXAMPLE
Composite number 329 (with sigma(329) = 384) is in sequence because sigma(329+sigma(329)) = sigma(713) = 768 = 2*384.
Prime 359 (with sigma(359) = 360) is in sequence because sigma(359+sigma(359)) = sigma(719) = 720 = 2*360.
MATHEMATICA
Select[Range[1500], DivisorSigma[1, # + DivisorSigma[1, #]] == 2 DivisorSigma[1, #] &] (* Michael De Vlieger, Aug 05 2021 *)
PROG
(Magma) [n:n in[1..10000] | SumOfDivisors(n+SumOfDivisors(n)) eq 2*SumOfDivisors(n)]
(PARI) select(n -> sigma(n+sigma(n))==2*sigma(n), [1..1000]) \\ Edward Jiang, Sep 05 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Sep 05 2014
STATUS
approved