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%I #9 Jan 04 2017 14:11:20
%S 0,1,2,2,3,3,4,4,4,0,5,8,11,13,15,18,20,22,24,1,5,8,11,13,15,18,20,22,
%T 24,2,6,9,11,14,16,18,21,23,24,2,6,9,11,14,16,18,21,23,24,3,6,9,11,14,
%U 16,18,21,23,24,3,6,9,11,14,16,18,21,23,24,4,7,9,12,15,16,19,21,23,25
%N ip(n): the number of primes not exceeding reverse(n).
%C "ip" is "pi" backwards.
%H Robert Israel, <a href="/A074719/b074719.txt">Table of n, a(n) for n = 1..10000</a>
%F ip(n) = pi(reverse(n)).
%e ip(12) = pi(21) = 8.
%p reverse:= proc(n) local L,d;
%p L:= convert(n,base,10);
%p add(10^(i-1)*L[-i],i=1..nops(L));
%p end proc:
%p map(numtheory:-pi @ reverse, [$1..100]); # _Robert Israel_, Jan 04 2017
%t rev[n_] := FromDigits[Reverse[IntegerDigits[n]]]; f[n_] := Module[{r, i}, r = 0; i = 1; While[Prime[i] <= n, i++ ]; i - 1]; Table[f[rev[n]], {n, 1, 100}]
%t PrimePi[FromDigits[Reverse[IntegerDigits[#]]]]&/@Range[80] (* _Harvey P. Dale_, May 28 2015 *)
%Y Cf. A000720, A004086.
%K base,easy,nonn,look
%O 1,3
%A _Joseph L. Pe_, Sep 27 2002
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