OFFSET
0,3
COMMENTS
a(n) is the determinant of the symmetric matrix (if(j<=floor((i+j)/2), Pell(j+1),
Pell(i+1)))_{0<=i,j<=n}, where Pell(n)=A000129(n).
FORMULA
a(n)=Product{k=0..n, sum{j=0..floor(k/2), binomial(k,2j)*2^j}}.
a(n) ~ c * (1+sqrt(2))^(n*(n+1)/2) / 2^(n+1), where c = 1.6982679851338713863950411843311686297311132648098280324748781109134... . - Vaclav Kotesovec, Jul 11 2015
EXAMPLE
a(3)=21 since det[1, 1, 1, 1; 1, 2, 2, 2; 1, 2, 5, 5; 1, 2, 5, 12]=21.
MATHEMATICA
Table[Product[Sum[Binomial[k, 2*j]*2^j, {j, 0, Floor[k/2]}], {k, 0, n}], {n, 0, 15}] (* Vaclav Kotesovec, Jul 11 2015 *)
Table[FullSimplify[Product[((1+Sqrt[2])^k + (1-Sqrt[2])^k)/2, {k, 0, n}]], {n, 0, 15}] (* Vaclav Kotesovec, Jul 11 2015 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Paul Barry, Feb 16 2011
STATUS
approved