[seqfan] A series for exp(Pi)

[Jaume Oliver i Lafont]

Dear Seqfans,

http://research.att.com/~njas/sequences/A166748
has a(0)=1, a(1)=6 and a(n)=(40-4*n+n^2)*a(n-2) for n>=2.
( See http://research.att.com/~njas/sequences/A039661 )

Maybe I should already know, but... how is the closed form of this
kind of recurrences found?

Thank you,
Jaume