

A099078


Numbers k such that pi(k).pi(k1) ... pi(3).pi(2) is prime (dot between numbers means concatenation).


3




OFFSET

1,1


COMMENTS

Number of digits of primes corresponding to the five known terms of this sequence are respectively 4, 21, 67, 605, 1633.


LINKS

Table of n, a(n) for n=1..6.
C. Rivera, Primes by Listing, The Prime Puzzles & Problems connection.
Eric Weisstein Integer Sequence Primes


EXAMPLE

5 is in the sequence because pi(5).pi(4).pi(3).pi(2) = 3221 is prime.


MAPLE

r:= 1: v:= 1: Res:= NULL:
for k from 3 to 6000 do
if isprime(k) then r:= r+1 fi;
v:= v + r*10^(1+ilog10(v));
if isprime(v) then Res:= Res, k fi
od:
Res; # Robert Israel, Nov 20 2018


MATHEMATICA

s = ""; Do[s = ToString[PrimePi[n]] <> s; k = ToExpression[s]; If[PrimeQ[k], Print[n]], {n, 2, 5235}] (* Ryan Propper, Aug 30 2005 *)


CROSSREFS

Cf. A046035, A099077, A099079, A099080.
Sequence in context: A082005 A273024 A273121 * A272836 A273575 A272848
Adjacent sequences: A099075 A099076 A099077 * A099079 A099080 A099081


KEYWORD

base,more,nonn


AUTHOR

Farideh Firoozbakht, Oct 23 2004


EXTENSIONS

One more term from Ryan Propper, Aug 30 2005


STATUS

approved



