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 A055377 a(n) = largest prime <= n/2. 3

%I

%S 2,2,3,3,3,3,5,5,5,5,7,7,7,7,7,7,7,7,11,11,11,11,13,13,13,13,13,13,13,

%T 13,17,17,17,17,19,19,19,19,19,19,19,19,23,23,23,23,23,23,23,23,23,23,

%U 23,23,29,29,29,29,31,31,31,31,31,31,31,31,31,31,31,31,37,37,37,37,37

%N a(n) = largest prime <= n/2.

%C Also largest prime factor of any composite <= n. E.g., a(15) = 7 since 7 is the largest prime factor of {4,6,8,9,10,12,14,15}, the composites <= 15.

%C Also largest prime dividing A025527(n) = n!/lcm[1,...,n]. [Comment from _Ray Chandler_, Apr 26 2007: Primes > n/2 don't appear as factors of A025527(n) since they appear once in n! and again in the denominator lcm[1,...,n]. Primes <= n/2 appear more times in the numerator than the denominator so they appear in the fraction.]

%C a(n) is the largest prime factor whose exponent in the factorization of n! is greater than 1. - _Michel Marcus_, Nov 11 2018

%H Michael De Vlieger, <a href="/A055377/b055377.txt">Table of n, a(n) for n = 4..10000</a>

%F a(n) = Max(gpf((n+2) mod k): 1 < k < (n+2) and k not prime), with gpf=A006530 (greatest prime factor). - _Reinhard Zumkeller_, Mar 27 2004

%F Where defined, that is for n > 2, a(A000040(n)) = A000040(A079952(n)). - _Peter Munn_, Sep 18 2017

%e n = 10, n! = 3628800, lcm[1,...,10] = 2520, A025527(10) = 1440 = 32*9*5 so a(7) = 5 (offset = 3).

%t Table[Prime@ PrimePi[n/2], {n, 4, 78}] (* _Michael De Vlieger_, Sep 21 2017 *)

%o (PARI) a(n) = precprime(n/2); \\ _Michel Marcus_, Sep 20 2017

%Y Cf. A000040, A000142, A003418, A006530, A007917, A025527, A060265, A079952, A079953.

%K nonn

%O 4,1

%A _Labos Elemer_, Jun 22 2000; _David W. Wilson_, Jun 10 2005

%E More terms from _James A. Sellers_, Jul 04 2000

%E Edited by _N. J. A. Sloane_ at the suggestion of _Andrew S. Plewe_, May 14 2007

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Last modified December 8 12:07 EST 2019. Contains 329862 sequences. (Running on oeis4.)