OFFSET
1,5
COMMENTS
As N increases, it appears that (sum_{k=1..N} a(k)) / (sum_{k=1..N} k) tends to 0.8.
LINKS
Pierre CAMI, Table of n, a(n) for n = 1..4000
MATHEMATICA
Table[k = 1; While[! PrimeQ[(k*2^n + 1)*k*2^n - 1], k = k + 2]; k, {n, 100}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Pierre CAMI, Mar 09 2011
STATUS
approved