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 A318169 Composite numbers k such that sigma_2(k) - 1 is a square, where sigma_2(k) = A001157(k) is the sum of squares of divisors of k. 0
 6, 40, 136, 2696, 3352, 46976, 223736, 5509736, 1915798072 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This property is shared with all the primes since sigma_2(p) = 1 + p^2. The values of sqrt(sigma_2(a(n)-1)) are 7, 47, 157, 3107, 3863, 54243, 257843, 6349657, 2207848187. Are there terms not of the form 2^k * p where p is prime? - David A. Corneth, Aug 20 2018 2*10^12 < a(10) <= 44463118771144. The terms 21687324345660824, 14524130539077100050485512, 287674439504279743204606472 (and others) of the form 2^k * p can be found by solving the quadratic Diophantine equation sigma_2(2^k) * (p^2 + 1) = x^2 + 1 for appropriate values of k. - Giovanni Resta, Aug 20 2018 LINKS MATHEMATICA sQ[n_] := IntegerQ[Sqrt[n]]; aQ[n_] := CompositeQ[n] && sQ[DivisorSigma[2, n]-1]; Select[Range[10000], aQ] PROG (PARI) forcomposite(n=2, 1e15, if( issquare(sigma(n, 2)-1), print1(n, ", "))) (MAGMA) [n: n in [2..6*10^6] |not IsPrime(n) and IsSquare(DivisorSigma(2, n)-1)]; // Vincenzo Librandi, Aug 22 2018 CROSSREFS Cf. A001157, A046655, A289290. Sequence in context: A229638 A210291 A089207 * A027777 A227013 A073773 Adjacent sequences:  A318166 A318167 A318168 * A318170 A318171 A318172 KEYWORD nonn,more AUTHOR Amiram Eldar, Aug 20 2018 STATUS approved

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Last modified February 24 22:04 EST 2020. Contains 332216 sequences. (Running on oeis4.)