269128937219domain: NAppears in sequencesa(n) = binomial(2n+1, n+1) - 1.at n=20A010763Primes of the form (2*k)!/(2*(k!)^2)-1.at n=3A112862a(n) is the numerator of tanh(Sum_{k=1..n-1} artanh(k/n)), where artanh is the inverse hyperbolic tangent function.at n=20A382257