A059788 - OEIS

%I #26 May 08 2021 22:58:20

%S 3,5,7,13,19,23,31,37,43,53,61,73,79,83,89,103,113,113,131,139,139,

%T 157,163,173,193,199,199,211,211,223,251,257,271,277,293,293,313,317,

%U 331,337,353,359,379,383,389,397,421,443,449,457,463,467,479,499,509,523

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

%C Also, smallest member of the first pair of consecutive primes such that between them is a composite number divisible by the n-th prime. - _Amarnath Murthy_, Sep 25 2002

%C Except for its initial term, A006992 is a subsequence based on iteration of n -> A151799(2n). The range of this sequence is a subset of A065091. - _M. F. Hasler_, May 08 2016

%H Zak Seidov, <a href="/A059788/b059788.txt">Table of n, a(n) for n = 1..1000</a>

%F a(n) = A007917(A100484(n)). - _R. J. Mathar_, May 08 2016

%e n=18: p(18)=61, so a(18) is the largest prime below 2*61=122, which is 113.

%p with(numtheory):

%p A059788 := proc(n)

%p     prevprime(2*ithprime(n)) ;

%p end proc:

%p seq(A059788(n),n=1..50) ; # _R. J. Mathar_, May 08 2016

%t a[n_] := Prime[PrimePi[2Prime[n]]]

%t NextPrime[2*Prime[Range[100]], -1] (* _Zak Seidov_, May 08 2016 *)

%o (PARI) a(n) = precprime(2*prime(n)); \\ _Michel Marcus_, May 08 2016

%Y Cf. A005382, A005383, A005384, A005385, A055496, A006992, A052248.

%K nonn

%O 1,1

%A _Labos Elemer_, Feb 22 2001