%I #20 Dec 22 2022 08:17:19
%S 1,25,304,3909,58299,907721
%N Number of distinct primes of type k + reverse(k) when k is a (2n - 1)-digit number.
%e 1 + reverse(1) = 2 is prime and for no other 1-digit number k, k + reverse(k) = 2k is prime, thus a(2*1-1) = a(1) = 1.
%o (PARI)
%o R(k)=fromdigits(Vecrev(digits(k)))
%o a(n)=my(m=2*n-1,u=Set([]),mmin=10^(m-1),mmax=10^m-1,card=0);for(m=mmin,mmax,y=m+R(m);if(isprime(y)&&!setsearch(u,y),u=Set(concat(u,y));card++));card
%Y Cf. A358985, A056964, A067030, A358986.
%K nonn,base,more
%O 1,2
%A _Jean-Marc Rebert_, Dec 16 2022