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# Ramanujan prime

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 Title: Ramanujan prime Author: World Heritage Encyclopedia Language: English Subject: Collection: Publisher: World Heritage Encyclopedia Publication Date:

### Ramanujan prime

In mathematics, a Ramanujan prime is a prime number that satisfies a result proven by Srinivasa Ramanujan relating to the prime-counting function.

## Contents

• Origins and definition 1
• Bounds and an asymptotic formula 2
• Generalized Ramanujan primes 3
• Ramanujan prime corollary 4
• References 5

## Origins and definition

In 1919, Ramanujan published a new proof of Bertrand's postulate which, as he notes, was first proved by Chebyshev. At the end of the two-page published paper, Ramanujan derived a generalized result, and that is:

\pi(x) - \pi(x/2) ≥ 1, 2, 3, 4, 5, ... for all x ≥ 2, 11, 17, 29, 41, ...  A104272 respectively,

where \pi(x) is the prime-counting function, equal to the number of primes less than or equal to x.

The converse of this result is the definition of Ramanujan primes:

The nth Ramanujan prime is the least integer Rn for which \pi(x) - \pi(x/2)n, for all xRn.

The first five Ramanujan primes are thus 2, 11, 17, 29, and 41. Equivalently,

Ramanujan primes are the least integers Rn for which there are at least n primes between x and x/2 for all xRn.

Note that the integer Rn is necessarily a prime number: \pi(x) - \pi(x/2) and, hence, \pi(x) must increase by obtaining another prime at x = Rn. Since \pi(x) - \pi(x/2) can increase by at most 1,

\pi(Rn) - \pi(Rn/2) = n.

## Bounds and an asymptotic formula

For all n ≥ 1, the bounds

2n ln 2n < Rn < 4n ln 4n

hold. If n > 1, then also

p2n < Rn < p3n

where pn is the nth prime number.

As n tends to infinity, Rn is asymptotic to the 2nth prime, i.e.,

Rn ~ p2n (n → ∞).

All these results were proved by Sondow (2009), except for the upper bound Rn < p3n which was conjectured by him and proved by Laishram (2010). The bound was improved by Sondow, Nicholson, and Noe (2011) to

R_n \le \frac{41}{47} \ p_{3n}

which is the optimal form of Rn < c·p3n since it is an equality for n = 5.

In a different direction, Axler showed that

R_n < p_{\lceil t\cdot n \rceil}

is optimal for t > 48/19, where \lceil\cdot \rceil is the ceiling function.

## Generalized Ramanujan primes

Given a constant c between 0 and 1, the nth c-Ramanujan prime is defined as the smallest integer Rc,n with the property that for any integer x ≥ Rc,n there are at least n primes between cx and x, that is, \pi(x) - \pi(cx) \ge n. In particular, when c = 1/2, the nth 1/2-Ramanujan prime is equal to the nth Ramanujan prime: R0.5,n = Rn.

For c = 1/4 and 3/4, the sequence of c-Ramanujan primes begins

R0.25,n = 2, 3, 5, 13, 17, ...  A193761,
R0.75,n = 11, 29, 59, 67, 101, ...  A193880.

It is known that, for all n and c, the nth c-Ramanujan prime Rc,n exists and is indeed prime. Also, as n tends to infinity, Rc,n is asymptotic to pn/(1 − c)

Rc,n ~ pn/(1 − c) (n → ∞)

where pn/(1 − c) is the \lfloorn/(1 − c)\rfloor th prime and \lfloor .\rfloor is the floor function.

## Ramanujan prime corollary

2p_{i-n} > p_i \text{ for } i>k \text{ where } k=\pi(p_k)=\pi(R_n)\, ,

i.e. pk is the kth prime and the nth Ramanujan prime.

This is very useful in showing the number of primes in the range [pk, 2*pi-n] is greater than or equal to 1. By taking into account the size of the gaps between primes in [pin,pk], one can see that the average prime gap is about ln(pk) using the following Rn/(2n) ~ ln(Rn).

Proof of Corollary:

If pi > Rn, then pi is odd and pi − 1 ≥ Rn, and hence π(pi − 1) − π(pi/2) = π(pi − 1) − π((pi − 1)/2) ≥ n. Thus pi − 1 ≥ pi−1 > pi−2 > pi−3 > ... > pin > pi/2, and so 2pin > pi.

An example of this corollary:

With n = 1000, Rn = pk = 19403, and k = 2197, therefore i ≥ 2198 and in ≥ 1198. The smallest i − n prime is pin = 9719, therefore 2pin = 2 × 9719 = 19438. The 2198th prime, pi, is between pk = 19403 and 2pin = 19438 and is 19417.

The left side of the Ramanujan Prime Corollary is the  A168421; the smallest prime on the right side is  A168425. The sequence  A165959 is the range of the smallest prime greater than pk. The values of \pi(R_n)\, are in the  A179196.

The Ramanujan Prime Corollary is due to John Nicholson.

Srinivasan's Lemma  states that pk-n < pk/2 if Rn = pk and n > 1. Proof: By the minimality of Rn, the interval (pk/2,pk] contains exactly n primes and hence pk-n < pk/2.