A071578 - OEIS

%I #16 Sep 20 2024 15:37:40

%S 0,1,2,2,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,4,5,5,5,5,

%T 5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,

%U 5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5

%N Number of iterations of Pi(n) needed to reach 1, where Pi(x) denotes the number of primes <= x.

%F a(n) = a(Pi(n))+1.

%F a(n) = A060197(n) - 2. - _Filip Zaludek_, Dec 10 2016

%t Table[Length[NestWhileList[PrimePi[#]&,n,#!=1&]]-1,{n,110}] (* _Harvey P. Dale_, Dec 24 2018 *)

%o (PARI) for(n=2,150,s=n; t=0; while(s!=1,t++; s=sum(i=2,s,isprime(i)); if(s==1,print1(t,","); ); ))

%Y Cf. A000720.

%K easy,nonn,changed

%O 1,3

%A _Benoit Cloitre_, May 31 2002