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 A331635 Numbers m such that -m^2 == m (mod sigma(m)) where sigma = A000203. 1
 1, 2, 3, 5, 7, 11, 13, 15, 17, 19, 20, 23, 24, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 90, 95, 97, 101, 103, 104, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 207, 211, 223, 224, 227, 229, 233, 239, 241, 251 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS All primes are terms; nonprime terms are 1, 15, 20, 24, 90, 95, 104, 207, 224, 287, 464, 588, 650, 1023, ... Equivalently: The m-th oblong number A002378(m) = m(m+1) is a multiple of sigma(m). - M. F. Hasler, Mar 04 2020 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 MAPLE filter:= n -> n^2 + n mod numtheory:-sigma(n) = 0: select(filter, [\$1..300]); # Robert Israel, Apr 12 2020 MATHEMATICA Select[Range[250], Divisible[# + #^2, DivisorSigma[1, #]] &] (* Amiram Eldar, Feb 26 2020 *) PROG (Magma) [1] cat [m: m in [1..251] | -m^2 mod SumOfDivisors(m) eq m]; (PARI) select( {is_A331635(n)=!(n*(n+1)%sigma(n))}, [1..200]) \\ M. F. Hasler, Mar 04 2020 CROSSREFS Cf. A000203. Sequence in context: A298536 A331784 A325162 * A320055 A072697 A102553 Adjacent sequences: A331632 A331633 A331634 * A331636 A331637 A331638 KEYWORD nonn AUTHOR Juri-Stepan Gerasimov, Feb 26 2020 EXTENSIONS a(1) = 1 from Amiram Eldar, Feb 26 2020 STATUS approved

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Last modified August 3 12:37 EDT 2024. Contains 374893 sequences. (Running on oeis4.)