OFFSET
3,1
LINKS
Andrew Howroyd, Minimal edge covers in the n-sun graph
Eric Weisstein's World of Mathematics, Minimal Edge Cover
Eric Weisstein's World of Mathematics, Sun Graph
FORMULA
a(n) = Sum_{i=0..n/2} Sum_{j=i..n/2} binomial(j,i)*A053530(i)*(2*binomial(n,2*j)*(n-j)^(j-i) + Sum_{k=1..(n-2*j)/3} n*binomial(j+k-1,j)*binomial(n-k-1,2*k+2*j-1)*(n-2*k-j)^(j-i)/k). - Andrew Howroyd, Aug 13 2017
MATHEMATICA
b[n_] := b[n] = n!*SeriesCoefficient[Exp[-x-x^2/2 + x*Exp[x]], {x, 0, n}];
a[n_] := Sum[b[i]*Sum[Binomial[j, i]*(2*Binomial[n, 2*j]*(n - j)^(j - i) + Sum[n*Binomial[j + k - 1, j]*Binomial[n - k - 1, 2*k + 2*j - 1]*(n - 2*k - j)^(j - i)/k, {k, 1, (n - 2*j)/3}]), {j, i, n/2}], {i, 0, n/2}];
Table[a[n], {n, 3, 24}] (* Jean-François Alcover, Oct 02 2017, after Andrew Howroyd *)
PROG
(PARI) \\ here b(n) is A053530
b(n)={Vec(serlaplace(exp(-x-1/2*x^2+x*exp(x + O(x^(n+1))))))[n+1]}
a(n) ={sum(i=0, n\2, b(i)*sum(j=i, n\2, binomial(j, i)*(2*binomial(n, 2*j)*(n-j)^(j-i) + sum(k=1, (n-2*j)\3, n*binomial(j+k-1, j)*binomial(n-k-1, 2*k+2*j-1)*(n-2*k-j)^(j-i)/k) )))} \\ Andrew Howroyd, Aug 13 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Eric W. Weisstein, Aug 10 2017
EXTENSIONS
a(6)-a(9) from Andrew Howroyd, Aug 11 2017
a(10)-a(24) from Andrew Howroyd, Aug 13 2017
STATUS
approved