A general form is R(R(p,a),b), where a>=2, b>=2 are integers (define R(p,b) = (b^p-1)/(b-1) = generalized repunit base b with length p)

These are the smallest prime R(p,b) with prime p>2 in all bases b<=1024:

https://mersenneforum.org/showpost.p...&postcount=228, for perfect power bases, there are no possible primes except a very low prime p because of algebra factors, there are only 32 bases b<=1024 remain with no known repunit (probable) primes: {185, 269, 281, 380, 384, 385, 394, 396, 452, 465, 511, 574, 598, 601, 629, 631, 632, 636, 711, 713, 759, 771, 795, 861, 866, 881, 938, 948, 951, 956, 963, 1005, 1015}, all searched to p=100K

The condition of R(R(p,a),b) is prime is R(p,a) is prime, and hence the bases a and b must not be perfect powers, however, for any integer pair (a,b), it is conjectured that only finitely many primes of the form R(R(p,a),b), since it is double exponent form, unlike R(p,b), which is conjectured to have infinitely many primes for all non-perfect-power bases b.