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
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
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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);
Comments