14316120domain: NAppears in sequencesNumbers that can be expressed as the difference of the squares of primes in exactly twenty-one distinct ways.at n=21A092017a(n) = denominator of Sum_{k=1..n} k^mu(n+1-k), where mu(m) = A008683(m).at n=24A130492a(n) is the denominator of Sum_{primes p < n} 1/(n-p).at n=25A305702