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A260648 Number of distinct prime divisors p of the n-th composite number c such that gpf(c - p) = p, where gpf = greatest prime factor (A006530). 1
1, 2, 0, 1, 2, 1, 1, 2, 0, 1, 1, 2, 1, 0, 1, 1, 1, 1, 2, 0, 1, 2, 2, 0, 1, 2, 0, 1, 1, 1, 1, 0, 1, 1, 2, 1, 0, 2, 1, 2, 1, 0, 1, 1, 0, 2, 2, 1, 1, 1, 0, 1, 1, 1, 2, 1, 1, 0, 1, 1, 2, 1, 1, 1, 0, 2, 1, 1, 1, 2, 0, 0, 2, 0, 1, 1, 2, 1, 0, 1, 2, 1, 1, 1, 1, 1, 1, 2, 0, 1, 1, 1, 1, 1, 0, 0, 1, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

a(n) gives the number of times that the n-th composite number occurs in A070229.

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

EXAMPLE

a(8) = 2 since the distinct prime divisors of A002808(8) = 15 are 3 and 5, A006530(15 - 3) = 3 and A006530(15 - 5) = 5, so all prime 3 and 5 are to be considered.

MAPLE

N:= 1000: # to consider composites <= N

f:= proc(c) local p, t;

   if isprime(c) then return NULL fi;

   nops(select(p -> max(numtheory:-factorset(c/p-1))<=p, numtheory:-factorset(c)))

end proc:

map(f, [$4..N]); # Robert Israel, May 02 2017

CROSSREFS

Cf. A002808 (composite), A006530 (gpf).

Sequence in context: A033781 A141803 A249147 * A127242 A025853 A228247

Adjacent sequences:  A260645 A260646 A260647 * A260649 A260650 A260651

KEYWORD

nonn

AUTHOR

Gionata Neri, Nov 12 2015

EXTENSIONS

a(87) corrected by Robert Israel, May 02 2017

STATUS

approved

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Last modified April 25 08:06 EDT 2018. Contains 303048 sequences. (Running on oeis4.)