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 A201879 Numbers n such that sigma_2(n) - n^2 is a square. 1
 1, 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 30, 31, 37, 41, 43, 47, 53, 59, 61, 67, 70, 71, 73, 79, 83, 89, 97, 101, 102, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Numbers n such that sum of the square of proper (or aliquot) divisors of n is a square. All primes are in this sequence. Nonprimes in the sequence are 1, 30, 70, 102, 282, 286, 646, 730, 920, 1242, ...  - Charles R Greathouse IV, Dec 06 2011 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 FORMULA {n: A067558(n) in A000290}. - R. J. Mathar, Dec 07 2011 EXAMPLE a(12)=30 because the aliquot divisors of 30 are  1, 2, 3, 5, 6, 10, 15, the sum of whose squares is 1^2 + 2^2 + 3^2 + 5^2 + 6^2 + 10^2 + 15^2 = 400 = 20^2. MAPLE A067558 := proc(n)     numtheory[sigma][2](n)-n^2 ; end proc: isA201879 := proc(n)     issqr(A067558(n)) ; end proc: for n from 1 to 300 do     if isA201879(n) then         printf("%d, ", n);     end if; end do: # R. J. Mathar, Dec 07 2011 MATHEMATICA Select[Range[400], IntegerQ[Sqrt[DivisorSigma[2, #]-#^2]]&] PROG (PARI) is(n)=issquare(sigma(n, 2)-n^2) \\ Charles R Greathouse IV, Dec 06 2011 CROSSREFS Cf. A000290, A001065, A001157, A073040. Sequence in context: A095959 A028905 A030059 * A327783 A319333 A326715 Adjacent sequences:  A201876 A201877 A201878 * A201880 A201881 A201882 KEYWORD nonn AUTHOR Michel Lagneau, Dec 06 2011 STATUS approved

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Last modified June 24 07:33 EDT 2021. Contains 345416 sequences. (Running on oeis4.)