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Number of distinct primes of type k + reverse(k) when k is a (2n - 1)-digit number.
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%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