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 A232355 Numbers n such that sigma(triangular(n)) = sigma(n)^2. 2
 1, 11, 695, 991, 2839, 3707, 9347, 10703, 12847, 27089, 42251, 56419, 74671, 115289, 168739, 191051, 219295, 233729, 280111, 300731, 326899, 353651, 430859, 611799, 642991, 661715, 1035827, 1116607, 1181579, 1234519, 1365491, 1485035, 1777099, 1854671, 1905875 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Subsequence of A116990. - Michel Marcus, Jun 13 2015 LINKS Amiram Eldar, Table of n, a(n) for n = 1..100 EXAMPLE 11 is in the sequence because sigma(11*12/2) = sigma(66) = 144 = 12^2 = sigma(11)^2. MATHEMATICA Select[Range@1000000, DivisorSigma[1, #]^2==DivisorSigma[1, (# (# + 1)/2)] &] (* Vincenzo Librandi, Jun 13 2015 *) PROG (PARI) isok(n) = sigma(n)^2 == sigma(n*(n+1)/2); \\ Michel Marcus, Nov 23 2013 (MAGMA) [n: n in [1..7*10^5] | SumOfDivisors(n*(n+1) div 2) eq SumOfDivisors(n)^2]; // Vincenzo Librandi, Jun 13 2015 CROSSREFS Cf. A000203 (sigma(n): sum of divisors of n), A000217 (triangular(n): = n*(n+1)/2). Cf. A074285 (sigma(triangular(n)), A072861 (sigma(n)^2). Cf. A116990 (indices of triangular numbers whose sum of divisors is square). Sequence in context: A195889 A195519 A218716 * A301732 A028457 A222653 Adjacent sequences:  A232352 A232353 A232354 * A232356 A232357 A232358 KEYWORD nonn AUTHOR Alex Ratushnyak, Nov 22 2013 EXTENSIONS More terms from Michel Marcus, Nov 23 2013 STATUS approved

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Last modified June 6 04:15 EDT 2020. Contains 334859 sequences. (Running on oeis4.)