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A061395 Let p be the largest prime factor of n; if p is the k-th prime then set a(n) = k; a(1) = 0 by convention. 110

%I

%S 0,1,2,1,3,2,4,1,2,3,5,2,6,4,3,1,7,2,8,3,4,5,9,2,3,6,2,4,10,3,11,1,5,

%T 7,4,2,12,8,6,3,13,4,14,5,3,9,15,2,4,3,7,6,16,2,5,4,8,10,17,3,18,11,4,

%U 1,6,5,19,7,9,4,20,2,21,12,3,8,5,6,22,3,2,13,23,4,7,14,10,5,24,3,6,9,11,15

%N Let p be the largest prime factor of n; if p is the k-th prime then set a(n) = k; a(1) = 0 by convention.

%C Records occur at the primes. - _Robert G. Wilson v_, Dec 30 2007.

%C For n > 1: length of n-th row in A067255. - _Reinhard Zumkeller_, Jun 11 2013

%C a(n) = the largest part of the partition having Heinz number n. We define the Heinz number of a partition p = [p_1, p_2, ..., p_r] as Product(p_j-th prime, j=1...r) (concept used by Alois P. Heinz in A215366 as an "encoding" of a partition). For example, for the partition [1, 1, 2, 4, 10] we get 2*2*3*7*29 = 2436. Example: a(20) = 3; indeed, the partition having Heinz number 20 = 2*2*5 is [1,1,3]. - _Emeric Deutsch_, Jun 04 2015

%H Álvar Ibeas, <a href="/A061395/b061395.txt">Table of n, a(n) for n = 1..100000</a> (first 1000 terms from Harry J. Smith)

%H <a href="/index/Pri#prime_indices">Index entries for sequences computed from indices in prime factorization</a>

%F A000040(a(n)) = A006530(n); a(n) = A049084(A006530(n)). - _Reinhard Zumkeller_, May 22 2003

%F A243055(n) = a(n) - A055396(n). - _Antti Karttunen_, Mar 07 2017

%e a(20) = 3 since the largest prime factor of 20 is 5, which is the 3rd prime.

%p with(numtheory):

%p a:= n-> `if`(n=1, 0, pi(max(factorset(n)[]))):

%p seq(a(n), n=1..100); # _Alois P. Heinz_, Aug 03 2013

%t Insert[Table[PrimePi[FactorInteger[n][[ -1]][[1]]], {n, 2, 120}], 0, 1] (* _Stefan Steinerberger_, Apr 11 2006 *)

%t f[n_] := PrimePi[ FactorInteger@n][[ -1, 1]]; Array[f, 94] (* _Robert G. Wilson v_, Dec 30 2007 *)

%o (PARI) { for (n=1, 1000, if (n==1, a=0, f=factor(n)~; p=f[1, length(f)]; a=primepi(p)); write("b061395.txt", n, " ", a) ) } \\ _Harry J. Smith_, Jul 22 2009

%o (Haskell)

%o a061395 = a049084 . a006530 -- _Reinhard Zumkeller_, Jun 11 2013

%o (Python)

%o from sympy import primepi, primefactors

%o def a(n): return 0 if n==1 else primepi(primefactors(n)[-1])

%o print [a(n) for n in xrange(1, 101)] # _Indranil Ghosh_, May 14 2017

%Y Cf. A006530, A055396, A061394, A133674, A243055.

%K easy,nice,nonn

%O 1,3

%A _Henry Bottomley_, Apr 30 2001

%E Definition reworded by _N. J. A. Sloane_, Jul 01 2008

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Last modified November 13 13:15 EST 2018. Contains 317149 sequences. (Running on oeis4.)