OFFSET
1,2
COMMENTS
The number of digits of a(n) is 1, 3, 15, 75, 376, 1881, 9407, 47036, 235180, 1175898, ....
-1/(4*a(n)) is the coefficient of x^0 of the minimal polynomial Psi(5^n,x) of cos(2*Pi/5^n). Hence 4*a(n)*Psi(5^n,x) is the integer polynomial with coefficient -1 of x^0. E.g., Psi(5,1)= x^2 + (1/2)*x -1/4, Psi(25,x)= x^10 + ... -1/1024. See A181875/A181876, A181877 and the W. Lang link under A181875.
FORMULA
a(n) = 2^(2*(5^(n-1) - 1)).
MATHEMATICA
Table[2^(2*(5^(n-1)-1)), {n, 1, 10}] (* G. C. Greubel, Jul 24 2017 *)
PROG
(Magma) [(2^(2*(5^((n-1)))-1)/2): n in [1..5]]; // Vincenzo Librandi, Apr 19 2011
(PARI) a(n)=1<<(2*(5^(n-1)-1)) \\ Charles R Greathouse IV, Jan 13 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wolfdieter Lang, Feb 24 2011
STATUS
approved