OFFSET
1,1
COMMENTS
Posted in response to a question from Dirk Boland.
EXAMPLE
a(7) = 12 is a term because Sum_{j=1..k} k^2/(2*j*(j+k)) = 13013256143/892371480, for k=12, 13013256143 is prime and this is the 7th such sum with a prime numerator.
MATHEMATICA
q[n_] := PrimeQ[Numerator[Sum[n^2/(2*j*(j + n)), {j, 1, n}]]]; Select[Range[1000], q] (* Amiram Eldar, Feb 19 2022 *)
PROG
(PARI) {ls=[]; for(k=1, 250, if(ispseudoprime(numerator(sum(j=1, k, k^2/(2*j*(j+k))))), ls=concat(ls, k))); print(ls)}
CROSSREFS
KEYWORD
nonn
AUTHOR
David Broadhurst, Dec 05 2006
EXTENSIONS
a(30)-a(49) from Amiram Eldar, Feb 19 2022
STATUS
approved