1,1

If one is trying to decide whether an n+1 digit binary number is prime, this is the largest prime for which one needs to test divisibility. For example a six digit number like 110101 must be below 64, so only primes up to 7 are needed to test divisibility. Compare with sequence A132153.

Harvey P. Dale, Table of n, a(n) for n = 1..1000

a(n) = A007917[A017910(n+1)]. - R. J. Mathar

seq(prevprime(floor(2^((n+1)*1/2))+1), n=1..40); # Emeric Deutsch

A017910 := proc(n) floor(2^(n/2)) ; end: A007917 := proc(n) prevprime(n+1) ; end: A133225 := proc(n) A007917(A017910(n+1)) ; end: seq(A133225(n), n=1..60) ; # R. J. Mathar

PrevPrim[n_] := Block[{k = n}, While[ !PrimeQ@k, k-- ]; k]; f[n_] := PrevPrim@ Floor@ Sqrt[2^(n + 1)]; Array[f, 42] (* Robert G. Wilson v *)

Table[Prime[PrimePi[2^((n + 1)/2)]], {n, 1, 50}] (* Stefan Steinerberger *)

lp[n_]:=Module[{c=2^((n+1)/2)}, If[PrimeQ[c], c, NextPrime[c, -1]]]; Array[lp, 50] (* Harvey P. Dale, Aug 25 2013 *)

Cf. A132153.

Sequence in context: A316075 A322429 A039894 * A240487 A066889 A214040

Adjacent sequences: A133222 A133223 A133224 * A133226 A133227 A133228

Anthony C Robin, Jan 03 2008

More terms from Stefan Steinerberger, R. J. Mathar, Robert G. Wilson v and Emeric Deutsch, Jan 06 2008

approved