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).

2, 7, 19, 487, 1637, 389047

Offset: 1

Author

Robert Israel, Jun 18 2024

Keywords

• nonn
• base
• more

Comments

a(7) > 7 * 10^9 if it exists.

Examples

			a(1) = 2 because 2 is prime while A004093(2) = 4 is not prime.
a(2) = 7 because 7 and A004093(7) = 41 are prime but A004093(41) = 28 is not.
a(3) = 19 because 19 and A004093(19) = 83 and A004093(83) = 661 are prime but A004093(661) = 2231 is not.
		

Crossrefs

Cf. A004093.

Programs

Maple

g:= proc(n) local t,L,i,j;
  t:= n;
  for i from 0 while isprime(t) do
    L:= convert(2*t,base,10);
    t:= add(L[-j]*10^(j-1),j=1..nops(L));
  od;
  i
end proc:
V:= Vector(6): count:= 0: p:= 1:
while count < 6 do
  p:= nextprime(p); v:= g(p);
if V[v] = 0 then V[v]:= p; count:= count+1 fi
od:
convert(V,list);