OFFSET
1,2
COMMENTS
Original name: Numbers n such that A259748(n) = 0.
LINKS
Danny Rorabaugh, Table of n, a(n) for n = 1..10000
Index entries for linear recurrences with constant coefficients, signature (2,-2,2,-2,2,-2,2,-2,2,-1).
FORMULA
A259748(a(n)) = Sum_{x*y: x,y in Z/a(n)Z, x<>y} = 0.
G.f.: x*(1+x^2)*(1+2*x^2-x^3+2*x^4-2*x^5+3*x^6+x^7) / ((1-x)^2*(1-x+x^2-x^3+x^4)*(1+x+x^2+x^3+x^4)). - Colin Barker, Aug 25 2016
MATHEMATICA
A[n_] := A[n] = Sum[a b, {a, 1, n}, {b, a + 1, n}] ; Select[Range[600], Mod[A[#], #] == 0 & ]
Rest@ CoefficientList[Series[x (1 + x^2) (1 + 2 x^2 - x^3 + 2 x^4 - 2 x^5 + 3 x^6 + x^7)/((1 - x)^2*(1 - x + x^2 - x^3 + x^4) (1 + x + x^2 + x^3 + x^4)), {x, 0, 61}], x] (* Michael De Vlieger, Aug 25 2016 *)
Select[Range[150], MemberQ[{1, 2, 5, 7, 10, 11, 13, 17, 19, 23}, Mod[#, 24]]&] (* or *) LinearRecurrence[{2, -2, 2, -2, 2, -2, 2, -2, 2, -1}, {1, 2, 5, 7, 10, 11, 13, 17, 19, 23}, 70] (* Harvey P. Dale, Jan 15 2022 *)
PROG
(PARI) Vec(x*(1+x^2)*(1+2*x^2-x^3+2*x^4-2*x^5+3*x^6+x^7)/((1-x)^2*(1-x+x^2-x^3+x^4)*(1+x+x^2+x^3+x^4)) + O(x^100)) \\ Colin Barker, Aug 25 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
José María Grau Ribas, Jul 04 2015
EXTENSIONS
Better name from Danny Rorabaugh, Oct 22 2015
STATUS
approved