A074719 ip(n): the number of primes not exceeding reverse(n).

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, 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, 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

Offset

1,3

Comments

"ip" is "pi" backwards.

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Formula

ip(n) = pi(reverse(n)).

Example

ip(12) = pi(21) = 8.

Maple

reverse:= proc(n) local L, d;
   L:= convert(n, base, 10);
   add(10^(i-1)*L[-i], i=1..nops(L));
end proc:
map(numtheory:-pi @ reverse, [$1..100]); # Robert Israel, Jan 04 2017

Mathematica

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}]
PrimePi[FromDigits[Reverse[IntegerDigits[#]]]]&/@Range[80] (* Harvey P. Dale, May 28 2015 *)

Crossrefs

Cf. A000720, A004086.

Sequence in context: A036041 A252759 A085654 * A079730 A035486 A282347

Adjacent sequences: A074716 A074717 A074718 * A074720 A074721 A074722

Keyword

base,easy,nonn,look

Author

Joseph L. Pe, Sep 27 2002

Status

approved