OFFSET
1,4
COMMENTS
a(n)=1 iff n is 2 or the lesser of twin primes (for n >= 3, n follows the sequence A001359).
Also the number of factors of n*(n+2) which are less than n. - Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Jul 02 2003
FORMULA
a(n) = ceiling( d(n*(n+2)) / 2 ) - 1, where d(n) = number of divisors of n (A000005). - Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Jul 02 2003
EXAMPLE
(4 + 1 + 4*1)/(4 - 1), (4 + 2 + 4*2)/(4 - 2), and (4 + 3 + 4*3)/(4 - 1) are integers, hence a(4)=3.
MAPLE
with(numtheory):A066728 := n->ceil(tau(n*(n+2))/2)-1;
CROSSREFS
KEYWORD
nonn
AUTHOR
Benoit Cloitre, Jan 15 2002
STATUS
approved