A152034 - OEIS

%I #12 Aug 16 2016 21:09:51

%S 79,769,9679,98999,995699,9975899,99967999,999548999,9999049999,

%T 99994169999,999989299999,9999954799999,99999904999999,

%U 999999778999999,9999999349999999,99999994999999999,999999971189999999,9999999950999999999,99999999632999999999

%N a(n) = largest n-digit prime p whose reversal is a prime q > p.

%H Robert Israel, <a href="/A152034/b152034.txt">Table of n, a(n) for n = 2..200</a>

%p revdigs:= proc(x) local L,i; L:= convert(x,base,10); add(L[-i]*10^(i-1),i=1..nops(L)) end proc:

%p f:= proc(n)

%p   local d,a,B,r;

%p   for d from floor(n/2) by -1 do

%p      B:= (10^d-1)*(1+10^(n-d));

%p      for a from 10^(n-2*d)-1 to 1 by -1 do

%p        r:= revdigs(a);

%p        if r > a and isprime(B+10^d*a) and isprime(B+10^d*r) then return B+10^d*a fi

%p      od

%p   od

%p end proc:

%p map(f, [$2..20]); # _Robert Israel_, Aug 16 2016

%t Do[ p = NextPrime[10^(n ), -1 ]; Do[ p1 = FromDigits[ Reverse[IntegerDigits[p]]]; If[PrimeQ[p1] && p1 > p, Print[{n, p}]; Break[]]; p = NextPrime[p, -1], {10^9}], {n, 2, 15}];

%Y Cf. A006567, A152014, A152033.

%K nonn,base

%O 2,1

%A _Zak Seidov_, Nov 20 2008

%E More terms from _Max Alekseyev_, May 03 2011