A228141 Numbers that are congruent to {1, 5} mod 20.

1, 5, 21, 25, 41, 45, 61, 65, 81, 85, 101, 105, 121, 125, 141, 145, 161, 165, 181, 185, 201, 205, 221, 225, 241, 245, 261, 265, 281, 285, 301, 305, 321, 325, 341, 345, 361, 365, 381, 385, 401, 405, 421, 425, 441, 445, 461, 465, 481, 485, 501, 505, 521, 525

Offset: 1

Author

Colin Barker, Aug 12 2013

Keywords

• nonn
• easy

Links

• Colin Barker, Table of n, a(n) for n = 1..1000
• Index entries for linear recurrences with constant coefficients, signature (1,1,-1).

Crossrefs

Cf. A216815, A122870, A228142.

Programs

Mathematica

Flatten[Table[20n + {1, 5}, {n, 0, 24}]] (* Alonso del Arte, Aug 12 2013 *)

PARI

Vec(x*(15*x^2+4*x+1)/((x-1)^2*(x+1)) + O(x^100))

Formula

a(n) = -3*(4+(-1)^n) + 10*n.

a(n) = a(n-1) + a(n-2) - a(n-3).

G.f.: x*(15*x^2+4*x+1) / ((x-1)^2*(x+1)).

E.g.f.: 15 + (10*x - 12)*exp(x) - 3*exp(-x). - David Lovler, Sep 05 2022