A126991 - OEIS

%I #10 Jan 25 2021 15:25:24

%S 3,5,7,11,13,19,19,23,29,31,37,43,47,47,53,61,61,67,73,79,83,89,89,97,

%T 101,103,113,113,113,113,137,139,139,149,157,157,167,167,173,181,181,

%U 181,199,199,211,211,211,229,233,239,241,241,251,257,263,271,271,281

%N Prime preceding the geometric mean of prime(n-2) and prime(n+2).

%t << NumberTheory`NumberTheoryFunctions` a = {}; Do[AppendTo[a,PreviousPrime[Sqrt[(Prime[x - 2])*(Prime[x + 2])]]], {x, 3, 100}]; a

%t Table[NextPrime[GeometricMean[{Prime[n-2],Prime[n+2]}],-1],{n,3,60}] (* _Harvey P. Dale_, May 14 2015 *)

%o (PARI) A126991(n)={ n=sqrtint(prime(n-2)*prime(n+2)); if(0==n%2, n--); while(!isprime(n), n-=2); n } /* then vector(50,n,A126991(n+2)) displays a list of values ; t=3;forprime(p=2,999, while(A126991(t)<p,t++);if(A126991(t)>p,print(p))) prints primes 2,17,41,59,... not in this sequence */ \\ _M. F. Hasler_, Jun 14 2007

%Y Cf. A126990, A126992, A124662.

%K nonn,easy,less

%O 3,1

%A _Artur Jasinski_, Jan 01 2007

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