OFFSET
1,2
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..50
MATHEMATICA
permcount[v_List] := Module[{m = 1, s = 0, k = 0, t}, For[i = 1, i <= Length[v], i++, t = v[[i]]; k = If[i > 1 && t == v[[i - 1]], k + 1, 1]; m *= t*k; s += t]; s!/m];
edges[v_List] := 4 Sum[Sum[GCD[v[[i]], v[[j]]], {j, 1, i - 1}], {i, 2, Length[v]}] + Sum[2 v[[i]] - 1, {i, 1, Length[v]}];
a[n_] := Module[{s = 0}, Do[s += permcount[2 p]*4^edges[p]*If[OddQ[n], n*16^Length[p], 1], {p, IntegerPartitions[Quotient[n, 2]]}]; s/n!];
Array[a, 25] (* Jean-François Alcover, Sep 12 2019, after Andrew Howroyd *)
PROG
(PARI)
permcount(v) = {my(m=1, s=0, k=0, t); for(i=1, #v, t=v[i]; k=if(i>1&&t==v[i-1], k+1, 1); m*=t*k; s+=t); s!/m}
edges(v) = {4*sum(i=2, #v, sum(j=1, i-1, gcd(v[i], v[j]))) + sum(i=1, #v, 2*v[i]-1)}
a(n) = {my(s=0); forpart(p=n\2, s+=permcount(2*Vec(p))*4^edges(p)*if(n%2, n*16^#p, 1)); s/n!} \\ Andrew Howroyd, Sep 17 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladeta Jovovic, Jan 24 2000
EXTENSIONS
Terms a(13) and beyond from Andrew Howroyd, Sep 17 2018
STATUS
approved