cp's OEIS Frontend

A345338 Integers whose Reverse And Add trajectory reaches its first prime after a record number of iterations (at least one iteration must be performed).

%I A345338 #16 Jul 10 2021 00:08:50
%S A345338 1,5,181,10031,1001320
%N A345338 Integers whose Reverse And Add trajectory reaches its first prime after a record number of iterations (at least one iteration must be performed).
%C A345338 a(6) > 10^9 (if it exists).
%C A345338 All numbers whose trajectory reaches a multiple of 3 or 11 before reaching a prime will never reach a prime.
%e A345338 a(3) = 181 because it takes 3 iterations (181 -> 362 -> 625 -> 1151 (prime)) to reach a prime, which is more than any smaller number.
%o A345338 (PARI) f(n) = my(t=n, c=1); while(!isprime(t+=fromdigits(Vecrev(digits(t)))), if(gcd(t, 33)>1, return(0)); c++); c;
%o A345338 lista(nn) = my(m); for(k=1, nn, if(f(k)>m, print1(k, ", "); m=f(k))); \\ _Jinyuan Wang_, Jun 15 2021
%o A345338 (Python)
%o A345338 from sympy import isprime
%o A345338 def ra(n): s = str(n); return int(s) + int(s[::-1])
%o A345338 def afind(limit):
%o A345338     record = 0
%o A345338     for k in range(limit+1):
%o A345338         m, i = ra(k), 1
%o A345338         while not isprime(m) and m%3 != 0 and m%11 != 0: m = ra(m); i += 1
%o A345338         if isprime(m) and i > record: record = i; print(k, end=", ")
%o A345338 afind(1234567) # _Michael S. Branicky_, Jul 03 2021
%Y A345338 Cf. A056964.
%K A345338 nonn,base,more
%O A345338 1,2
%A A345338 _Daniel Starodubtsev_, Jun 14 2021