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A088627 Let 2n = r*s. Then a(n) = number of primes of the form r+s (r= 1 and s = 2n contributes 1 to the count if 2n+1 is prime). 6
1, 1, 2, 0, 2, 2, 0, 1, 2, 0, 2, 1, 0, 2, 4, 0, 1, 2, 0, 2, 4, 0, 1, 1, 0, 2, 1, 0, 2, 4, 0, 0, 2, 0, 4, 2, 0, 1, 4, 0, 2, 2, 0, 2, 3, 0, 0, 1, 0, 2, 4, 0, 1, 2, 0, 2, 2, 0, 1, 3, 0, 0, 2, 0, 4, 3, 0, 1, 3, 0, 1, 0, 0, 2, 3, 0, 2, 2, 0, 1, 2, 0, 1, 3, 0, 2, 2, 0, 1, 3, 0, 1, 1, 0, 4, 2, 0, 2, 4, 0, 1, 2, 0, 1, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
Only even numbers yield primes hence odd numbers are not considered.
There is an upper bound: if n has only k different prime divisors > 2, then a(n) <= 2^k. - Matthias Engelhardt, Jan 05 2004
LINKS
EXAMPLE
a(9) = 2 18 = 1*18, 1+18= 19 and 18 = 2*9, 2+9 = 11, two primes arise.
CROSSREFS
Cf. A091350.
Sequence in context: A164273 A307694 A106277 * A230205 A334841 A024713
KEYWORD
nonn
AUTHOR
Amarnath Murthy, Oct 19 2003
EXTENSIONS
More terms from Matthias Engelhardt, Jan 05 2004
More terms from David Wasserman, Aug 15 2005
STATUS
approved

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Last modified April 13 02:05 EDT 2024. Contains 371639 sequences. (Running on oeis4.)