OFFSET
1,24
COMMENTS
Number of tangency points on all circles with radius sqrt(2) and center (p,q) where p and q are prime, p + q = 2n and p <= q. - Wesley Ivan Hurt, Aug 10 2020
LINKS
Eric Weisstein's World of Mathematics, Goldbach Partition
Wikipedia, Goldbach's conjecture
FORMULA
a(n) = Sum_{i=2..n-1} c(i-1) * c(2*n-i-1) * c(i+1) * c(2*n-i+1), where c is the prime characteristic (A010051).
a(n) = Sum_{i=3..n} c(i) * c(2*n-i) * c(i-2) * c(2*n-i+2), where c is the prime characteristic (A010051). - Wesley Ivan Hurt, Aug 10 2020
EXAMPLE
a(5) = 1; 2*5 = 10 has two Goldbach partitions, (p,q) = (3,7) and (s,t) = (5,5) where 3,5 are twin primes and 5,7 are twin primes. This makes exactly 1 such pair.
MATHEMATICA
Table[Sum[(PrimePi[i - 1] - PrimePi[i - 2]) (PrimePi[2 n - i - 1] - PrimePi[2 n - i - 2]) (PrimePi[i + 1] - PrimePi[i]) (PrimePi[2 n - i + 1] - PrimePi[2 n - i]), {i, 2, n - 1}], {n, 100}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Wesley Ivan Hurt, Apr 03 2018
STATUS
approved