%I #50 Mar 26 2024 05:40:37
%S 0,1,2,0,3,0,4,0,0,0,5,0,6,0,0,0,7,0,8,0,0,0,9,0,0,0,0,0,10,0,11,0,0,
%T 0,0,0,12,0,0,0,13,0,14,0,0,0,15,0,0,0,0,0,16,0,0,0,0,0,17,0,18,0,0,0,
%U 0,0,19,0,0,0,20,0,21,0,0,0,0,0,22,0,0,0,23,0,0,0,0,0,24,0,0,0
%N a(n) = pi(n) if n is prime, otherwise 0.
%C pi(n) is the prime counting function, A000720.
%C Equals row sums of triangle A143541. - _Gary W. Adamson_, Aug 23 2008
%H Reinhard Zumkeller, <a href="/A049084/b049084.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = pi(n)*(pi(n) - pi(n-1)), pi = A000720. - _Reinhard Zumkeller_, Nov 30 2003
%F a(n) = A000720(n*A010051(n)). - _Labos Elemer_, Jan 09 2004
%F a(n) = A000720(n)*A010051(n). - _R. J. Mathar_, Mar 01 2011
%p A049084 := proc(n)
%p local i;
%p if isprime(n) then
%p for i from 1 do
%p if ithprime(i) = n then
%p return i;
%p end if;
%p end do;
%p else
%p return 0 ;
%p fi;
%p end proc:
%p seq(A049084(n),n=1..120) ;
%t Table[PrimePi[n] * Boole[PrimeQ[n]], {n, 92}] (* _Jean-François Alcover_, Nov 07 2011, after _R. J. Mathar_ *)
%t Table[If[PrimeQ[n],PrimePi[n],0],{n,100}] (* _Harvey P. Dale_, Jan 09 2022 *)
%o (Haskell)
%o import Data.List (unfoldr)
%o a049084 n = a049084_list !! (fromInteger n - 1)
%o a049084_list = unfoldr x (1, 1, a000040_list) where
%o x (i, z, ps'@(p:ps)) | i == p = Just (z, (i + 1, z + 1, ps))
%o | i /= p = Just (0, (i + 1, z, ps'))
%o -- _Reinhard Zumkeller_, Apr 17 2012, Mar 31 2012, Sep 15 2011
%o (PARI) a(n)=if(isprime(n),primepi(n),0) \\ _Charles R Greathouse IV_, Jan 08 2013
%Y a(n) = A091227(A091202(n)).
%Y Cf. A143541.
%K nonn,easy
%O 1,3
%A _N. J. A. Sloane_
%E Name clarified by _Alonso del Arte_, Feb 07 2020 at the suggestion of _David A. Corneth_