OFFSET
1,1
COMMENTS
After a(1) = 5, when is a(n) prime?
FORMULA
a(n) = SUM[i=1,n] 2^(n+1).
EXAMPLE
a(1) = 5 because 101 (base 2) = 5 (base 10).
a(2) = 161 because 10100001 (base 2) = 161 (base 10).
a(3) = 164865 because 101000010000000001 (base 2) = 164865 (base 10).
MATHEMATICA
Table[FromDigits[Flatten[Join[{1}, Table[Join[PadRight[{}, n^2, 0], {1}], {n, i}]]], 2], {i, 8}] (* Harvey P. Dale, Mar 30 2016 *)
CROSSREFS
KEYWORD
base,easy,nonn
AUTHOR
Jonathan Vos Post, Apr 16 2006
STATUS
approved