cp's OEIS Frontend

A373765 a(n) is the first number that starts a sequence of exactly n primes x(1), ..., x(n) where x(i+1) = A004093(x(i)) is the digit reversal of 2 * x(i).

%I A373765 #9 Jun 23 2024 22:04:31
%S A373765 2,7,19,487,1637,389047
%N A373765 a(n) is the first number that starts a sequence of exactly n primes x(1), ..., x(n) where x(i+1) = A004093(x(i)) is the digit reversal of 2 * x(i).
%C A373765 a(7) > 7 * 10^9 if it exists.
%e A373765 a(1) = 2 because 2 is prime while A004093(2) = 4 is not prime.
%e A373765 a(2) = 7 because 7 and A004093(7) = 41 are prime but A004093(41) = 28 is not.
%e A373765 a(3) = 19 because 19 and A004093(19) = 83 and A004093(83) = 661 are prime but A004093(661) = 2231 is not.
%p A373765 g:= proc(n) local t,L,i,j;
%p A373765   t:= n;
%p A373765   for i from 0 while isprime(t) do
%p A373765     L:= convert(2*t,base,10);
%p A373765     t:= add(L[-j]*10^(j-1),j=1..nops(L));
%p A373765   od;
%p A373765   i
%p A373765 end proc:
%p A373765 V:= Vector(6): count:= 0: p:= 1:
%p A373765 while count < 6 do
%p A373765   p:= nextprime(p); v:= g(p);
%p A373765 if V[v] = 0 then V[v]:= p; count:= count+1 fi
%p A373765 od:
%p A373765 convert(V,list);
%Y A373765 Cf. A004093.
%K A373765 nonn,base,more
%O A373765 1,1
%A A373765 _Robert Israel_, Jun 18 2024