A350782 - OEIS

A350782

a(n) is the number of pairs of primes (p,q), p < q, such that (p + q)/2 = n!.

1, 5, 18, 60, 315, 1615, 9928, 70437, 637504, 5829386, 64647125, 722750400

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OFFSET

3,2

COMMENTS

a(n) is the number of pairs of primes that are equidistant from n!.

Equivalently, a(n) is the number of positive integers d such that n! - d and n! + d are primes. For the smallest such d, iff there are no primes in the open interval (n! - d, n! + d), then n is a term in A053709, n! is a term in A053710, and d is a term in A053711.

LINKS

Table of n, a(n) for n=3..14.

FORMULA

a(n) = A002375(n!).

EXAMPLE

For n = 4, n! = 24, from which 5 pairs of primes are equidistant; in order of increasing distance, these are (19, 29), (17, 31), (11, 37), (7, 41), and (5, 43), so a(4) = 5. (A prime (23) lies between 19 and 29, so n=4 is not a term of A053709.)

For n = 5, n! = 120, from which 18 pairs of primes are equidistant: (113, 127), (109, 131), (103, 137), (101, 139), (89, 151), (83, 157), (73, 167), (67, 173), (61, 179), (59, 181), (47, 193), (43, 197), (41, 199), (29, 211), (17, 223), (13, 227), (11, 229), and (7, 233), so a(5) = 18. (The pair least distant from 120 is (113, 127), and there are no primes between 113 and 127, so n = 5 is a term of A053709, n! = 120 is a term of A053710, and d = 120 - 113 = 127 - 120 = 7 is a term of A053711.)

MATHEMATICA

Table[Length@IntegerPartitions[2n!, 2, Prime@Range@PrimePi[2n!]], {n, 3, 9}] (* Giorgos Kalogeropoulos, Jan 16 2022 *)

PROG

(Python)

from sympy import isprime, nextprime, factorial

def A350782(n):

m, p, c = factorial(n), 3, 0

while p <= m:

if isprime(2*m-p):

c += 1

p = nextprime(p)

return c # Chai Wah Wu, Jan 16 2022