OFFSET
1,1
COMMENTS
a(2)=a(3)=a(4) as every odd fourth power is == 1 mod (2^4).
FORMULA
Empirical g.f.: x*(81920*x^7 -75776*x^6 -45184*x^5 +43696*x^4 -2840*x^3 -2530*x^2 +415*x -16) / ((x-1)*(2*x-1)*(4*x-1)*(8*x-1)*(16*x-1)). [Colin Barker, Feb 05 2013]
EXAMPLE
a(5) = 2401 = 7^4 and 7^4 -1 = (2^5)*3*5^2.
a(6) = 50625 = 15^4 and 15^4 -1 = (2^6)*7*113.
CROSSREFS
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Sep 19 2003
EXTENSIONS
Corrected and extended by Ray Chandler, Oct 04 2003
STATUS
approved