login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A074719 ip(n): the number of primes not exceeding reverse(n). 3
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 (list; graph; refs; listen; history; text; internal format)
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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 28 13:24 EDT 2020. Contains 337393 sequences. (Running on oeis4.)