login
A363967
Numbers whose divisors can be partitioned into two disjoint sets whose both sums are squares.
1
1, 3, 9, 22, 27, 30, 40, 63, 66, 70, 81, 88, 90, 94, 115, 119, 120, 138, 153, 156, 170, 171, 174, 184, 189, 190, 198, 210, 214, 217, 232, 264, 265, 270, 280, 282, 310, 318, 322, 323, 343, 345, 357, 360, 364, 376, 382, 385, 399, 400, 414, 416, 462, 468, 472, 495, 497
OFFSET
1,2
COMMENTS
If one of the two sets is empty then the term is a number whose sum of divisors is a square (A006532).
If k is a number such that (6*k)^2 is the sum of a twin prime pair (A037073), then (18*k^2)^2 - 1 is a term.
3 is the only prime term.
LINKS
EXAMPLE
9 is a term since its divisors, {1, 3, 9}, can be partitioned into the two disjoint sets, {1, 3} and {9}, whose sums, 1 + 3 = 4 = 2^2 and 9 = 3^2, are both squares.
MATHEMATICA
sqQ[n_] := IntegerQ[Sqrt[n]]; q[n_] := Module[{d = Divisors[n], s, p}, s = Total[d]; p = Position[Rest @ CoefficientList[Product[1 + x^i, {i, d}], x], _?(# > 0 &)] // Flatten; AnyTrue[p, sqQ[#] && sqQ[s - #] &]]; Select[Range[500], q]
CROSSREFS
Subsequence of A333911.
A006532 is a subsequence.
Similar sequences: A333677, A360694.
Sequence in context: A098980 A007647 A247182 * A318807 A063586 A131477
KEYWORD
nonn
AUTHOR
Amiram Eldar, Jun 30 2023
STATUS
approved