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A185279 a(n) = number of ways that one can write n as the sum of two positive integers such that i) the integers are relatively prime to n but ii) the integers are not themselves prime. 3
0, 1, 0, 0, 1, 0, 1, 0, 1, 1, 1, 0, 2, 0, 1, 1, 2, 0, 3, 0, 1, 1, 3, 0, 3, 1, 1, 1, 5, 0, 6, 0, 2, 2, 3, 1, 7, 0, 3, 1, 8, 0, 9, 1, 1, 2, 9, 0, 8, 1, 3, 2, 11, 0, 7, 1, 4, 3, 13, 0, 14, 1, 3, 4, 8, 1, 15, 1, 6, 1, 16, 0, 17, 3, 2, 4, 11, 1, 18, 0, 7, 4, 19, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,13

COMMENTS

These might be called "relative Goldbach partitions."

This sequence was first discovered by my student Houston Hutchinson.

We became interested in this sequence when looking at Goldbach Partitions thus at first we only considered the even numbered terms.  The graph of the even values of a(n) looks like Goldbach's comet except with an exponential appearance rather than a logarithmic appearance.  We give a formula for the even values in the formula section.

Sequence A141095 has the terms for even n.

LINKS

T. D. Noe, Table of n, a(n) for n = 1..5000

FORMULA

For even n >= 4, denote the number of Goldbach partitions that have distinct primes by g(n), denote the totient of n by t(n), and denote the primes less than n that are NOT factors of n by p(n). Then a(n) = g(n)- p(n) + t(n)/2.

a(n) = Sum_{i=1..floor(n/2)} [GCD(i, n-i) = 1] * c(i) * c(n-i), where c is the characteristic function of nonprimes (A005171) and [ ] is the Iverson bracket. - Wesley Ivan Hurt, Dec 08 2020

EXAMPLE

a(34) is the first even term with value greater than 1.  The number 34 = 33 + 1 and 25 + 9.  The latter sums meet the requirements listed in the definition. For odd n greater than 3, a(n) will always be at least 1 since 1 + (n - 1) is a sum that satisfies the definition.  For example a(5) = 1 since 5 = 1 + 4.

MATHEMATICA

Table[Length[Select[Range[n/2], ! PrimeQ[#] && ! PrimeQ[n - #] && GCD[#, n - #] == 1 &]], {n, 100}] (* T. D. Noe, Dec 05 2013 *)

PROG

(Sage)

def A185279(n):

    return sum(1 for i in (1..n//2) if all(gcd(j, n) == 1 and not is_prime(j) for j in (i, n-i))) # D. S. McNeil, Mar 05 2011

CROSSREFS

Cf. A005171, A141095.

Sequence in context: A202177 A029360 A337541 * A088432 A329747 A304455

Adjacent sequences:  A185276 A185277 A185278 * A185280 A185281 A185282

KEYWORD

nonn

AUTHOR

Jason Holland, Feb 19 2011

STATUS

approved

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Last modified June 24 18:46 EDT 2021. Contains 345419 sequences. (Running on oeis4.)