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
KEYWORD
AUTHOR
Joseph L. Pe, Sep 27 2002
STATUS
approved