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Palindromic primes q derived from palindromes p such that pi(p) = q.
5

%I #15 Sep 12 2019 01:54:29

%S 2,2,3,3,5,11,101,151,757,797,72727,3485843,7362637,753535357,

%T 32792329723,1644209024461,9600458540069,125319848913521,

%U 164957666759461

%N Palindromic primes q derived from palindromes p such that pi(p) = q.

%C From a suggestion from _Zak Seidov_, Feb 02 2005.

%C a(16) > pi(32*10^12). - _Donovan Johnson_, Dec 03 2009

%t NextPalindrome[n_] := Block[ {l = Floor[ Log[10, n] + 1], idn = IntegerDigits[n]}, If[ Union[ idn] == {9}, Return[n + 2], If[l < 2, Return[n + 1], If[ FromDigits[ Reverse[ Take[ idn, Ceiling[l/2]]]] FromDigits[ Take[ idn, -Ceiling[l/2]]], FromDigits[ Join[ Take[ idn, Ceiling[l/2]], Reverse[ Take[ idn, Floor[l/2]]] ]], idfhn = FromDigits[ Take[ idn, Ceiling[l/2]]] + 1; idp = FromDigits[ Join[ IntegerDigits[ idfhn], Drop[ Reverse[ IntegerDigits[ idfhn]], Mod[l, 2]]]] ]]]];

%t p = 0; a = {}; Do[p = NextPalindrome[p]; q = PrimePi[p]; If[PrimeQ[q], r = IntegerDigits[q]; If[Reverse[r] == r, Print[{p, q}]; AppendTo[a, q]]], {n, 10^6}]; a

%t palQ[n_] := Reverse[x = IntegerDigits[n]] == x; t = {}; Do[If[palQ[n] && PrimeQ[y = PrimePi[n]] && palQ[y], AppendTo[t, y]], {n,10^6}]; t (* _Jayanta Basu_, Jun 24 2013 *)

%Y Cf. A046941, A046942, A103357, A103358, A103402.

%K nonn,base,more

%O 1,1

%A _Robert G. Wilson v_, Feb 03 2005

%E a(15) from _Donovan Johnson_, Dec 03 2009

%E a(16)-a(17) from _Chai Wah Wu_, Sep 04 2019

%E a(18)-a(19) from _Giovanni Resta_, Sep 12 2019