A069577 Smallest prime p such that pi(n) <= pi(p)*2, where pi(n) is the number of primes <= n, A000720.

2, 2, 2, 3, 3, 3, 3, 3, 3, 5, 5, 5, 5, 5, 5, 7, 7, 7, 7, 7, 7, 11, 11, 11, 11, 11, 11, 11, 11, 13, 13, 13, 13, 13, 13, 13, 13, 13, 13, 17, 17, 17, 17, 17, 17, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 23, 23, 23, 23, 23, 23, 23, 23, 29, 29

Offset

2,1

Comments

a(n) separates the primes <= n in two sets {q<=a(n)} and {q<a(n)<=n} which differ in size by not more than 1.

Links

Table of n, a(n) for n=2..68.

Example

Primes <= 20: {2,3,5,7,11,13,17,19}, therefore pi(20)=8 and from 8<=pi(7)*2=4*2 follows a(20)=7.

Crossrefs

Cf. A007917, A000720, A000040.

Sequence in context: A070564 A072358 A074795 * A282426 A318781 A375834

Adjacent sequences: A069574 A069575 A069576 * A069578 A069579 A069580

Keyword

nonn

Author

Reinhard Zumkeller, Apr 18 2002

Status

approved