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 A289738 Numbers k whose sum of divisors equals the sum of divisors of 2*k-1. 1
 1, 6, 348, 496, 1420, 1854, 4674, 5352, 6424, 13545, 21126, 28210, 37336, 57645, 84370, 95526, 109648, 116865, 140056, 150366, 163450, 176826, 215430, 305900, 321496, 330858, 517914, 558304, 590790, 617260, 682746, 742518, 888550, 927336, 952938, 1058344, 1096758 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Most of the terms in this sequence are even. Up to 10^7 there are 102 terms out of which 7 are odd. There are two terms, 6 and 496, for which (2*k - 1) is a prime number. LINKS Amiram Eldar, Table of n, a(n) for n = 1..1000 EXAMPLE 6 is in the sequence because the sum of divisors of 6: (1 + 2 + 3 + 6 = 12); equals the sum of divisors of 11 = 2*6 - 1: (1 + 11 = 12). 348 is in the sequence because the sum of divisors of 348: (1 + 2 + 3 + 4 + 6 + 12 + 29 + 58 + 87 + 116 + 174 + 348 = 840); equals the sum of divisors of (2*348 - 1 = 695): (1 + 5 + 139 + 695 = 840). MAPLE with(numtheory): select(t -> sigma(t) = sigma(2*t-1), [\$1..10^6]); MATHEMATICA Select[Range[10^7], DivisorSigma[1, #] == DivisorSigma[1, 2 # - 1] &] PROG (PARI) for (n = 1,  1e7, (sigma(n)==sigma(2*n-1)) && print1(n ", ")); (MAGMA) [n : n in [1..10^6] | SumOfDivisors(n) eq SumOfDivisors(2*n-1)]; CROSSREFS Cf. A000203, A005101, A272553, A275370 (odd terms). Sequence in context: A212490 A047941 A229501 * A211089 A221923 A000409 Adjacent sequences:  A289735 A289736 A289737 * A289739 A289740 A289741 KEYWORD nonn,changed AUTHOR K. D. Bajpai, Jul 10 2017 STATUS approved

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Last modified May 28 17:37 EDT 2020. Contains 334684 sequences. (Running on oeis4.)