OFFSET
1,1
COMMENTS
Conjecture: every odd prime is a factor of at least one number in this sequence. - J. Lowell, Apr 17 2014
a(n) written in base 2 is A138831(n) which is also a member of A138148 (cyclops numbers with binary digits only), assuming there are no odd perfect numbers. - Omar E. Pol, Oct 24 2016 [clarified on Nov 03 2018].
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..15
FORMULA
a(n) = A000396(n) - 1.
MATHEMATICA
(# (# + 1))/2 & /@ Select[2^Range[100] - 1, PrimeQ] - 1 (* G. C. Greubel, Oct 24 2016, based on Harvey P. Dale code from A000396 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Omar E. Pol, Nov 27 2007
STATUS
approved