A126990 - OEIS

%I #19 Mar 19 2020 06:18:52

%S 3,3,7,7,13,13,19,23,23,31,31,37,43,47,47,53,61,61,67,73,73,83,89,89,

%T 97,103,103,109,113,113,131,131,139,139,151,151,157,167,167,173,181,

%U 181,193,193,199,199,211,223,229,233,233,241,241,251,257,263,271,271,277

%N Largest prime preceding geometric mean of prime(n) and prime(n+2).

%C With duplicates removed, seems to be a subsequence of A105399 and A105792. - _M. F. Hasler_, Jun 14 2007

%D P. Ribenboim, The Little Book of Bigger Primes, Springer-Verlag NY 2004.

%H Amiram Eldar, <a href="/A126990/b126990.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A007917(A073273(n)). - _Michel Marcus_, Nov 07 2013

%t << NumberTheory`NumberTheoryFunctions` a = {}; Do[AppendTo[a,PreviousPrime[Sqrt[(Prime[x])*(Prime[x + 2])]]], {x, 1, 100}]; a

%o (PARI) A126990(n)={ n=sqrtint(prime(n)*prime(n+2)); if( 0==n%2, n--); while(!isprime(n), n-=2); n } /* then vector(50,n,A126990(n)) displays a list of values, _M. F. Hasler_, Jun 14 2007 */

%o (PARI) a(n)= precprime(sqrtint(prime(n)*prime(n+2))); \\ _Michel Marcus_, Nov 07 2013

%Y Cf. A007917, A073273.

%K nonn

%O 1,1

%A _Artur Jasinski_, Jan 01 2007

%E Edited by _M. F. Hasler_, Jun 14 2007

%E Definition changed so that offset is now 1 by _Michel Marcus_, Nov 07 2013