A151980 Numbers n such that n^2 - n is divisible by 20.

0, 1, 5, 16, 20, 21, 25, 36, 40, 41, 45, 56, 60, 61, 65, 76, 80, 81, 85, 96, 100, 101, 105, 116, 120, 121, 125, 136, 140, 141, 145, 156, 160, 161, 165, 176, 180, 181, 185, 196, 200, 201, 205, 216, 220, 221, 225, 236, 240, 241, 245, 256, 260, 261, 265, 276, 280, 281, 285, 296

Offset: 1

Author

N. J. A. Sloane, Aug 23 2009

Keywords

• nonn
• easy

Links

• Index entries for linear recurrences with constant coefficients, signature (1,0,0,1,-1).

Programs

Mathematica

Select[Range[0,300],Divisible[#^2-#,20]&] (* or *) LinearRecurrence[ {1,0,0,1,-1},{0,1,5,16,20},60] (* Harvey P. Dale, Aug 16 2020 *)

Formula

G.f.: x^2*(1+4*x+11*x^2+4*x^3)/((1-x)^2*(1+x)*(1+x^2)). [Colin Barker, Jan 14 2012]

a(n) = a(n-1)+a(n-4)-a(n-5). [Colin Barker, Nov 29 2012]