A184285 Number of distinct n X 2 toroidal 0..4 arrays.

15, 175, 2635, 49075, 976887, 20349075, 435970995, 9536816575, 211927736135, 4768373047015, 108372083629275, 2483526896194175, 57312158484825735, 1330460821533203275, 31044085821533856483, 727595761432647754075, 17119900268689323874095

Offset: 1

Author

R. H. Hardin, Jan 10 2011

Keywords

• nonn

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..100

Crossrefs

Column 2 of A184288.

Programs

Mathematica

T[n_, k_] := (1/(n*k))*Sum[Sum[EulerPhi[c]*EulerPhi[d]*5^(n*k/LCM[c, d]), {d, Divisors[k]}], {c, Divisors[n]}]; a[n_] := T[n, 2]; Array[a, 17] (* Jean-François Alcover, Nov 01 2017, after Andrew Howroyd *)

PARI

a(n) = my(k=2); (1/(n*k)) * sumdiv(n, c, sumdiv(k, d, eulerphi(c) * eulerphi(d) * 5^(n*k/lcm(c, d)))); \\ Michel Marcus, Nov 01 2017

Extensions

Terms a(12) and beyond from Andrew Howroyd, Sep 27 2017