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:
- ≥ 1, 2, 3, 4, 5, ... for all x ≥ 2, 11, 17, 29, 41, ... A104272 respectively,
where 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 ≥ n, for all x ≥ Rn.
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 x ≥ Rn.
Note that the integer Rn is necessarily a prime number: and, hence, must increase by obtaining another prime at x = Rn. Since can increase by at most 1,
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
which is optimal since it is an equality for n = 5.
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, . 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 n/(1 − c)th prime and is the floor function.
Ramanujan prime corollary
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 [pi−n,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 > ... > pi−n > pi/2, and so 2pi−n > pi.
An example of this corollary:
With n = 1000, Rn = pk = 19403, and k = 2197, therefore i ≥ 2198 and i−n ≥ 1198. The smallest i − n prime is pi−n = 9719, therefore 2pi−n = 2 × 9719 = 19438. The 2198th prime, pi, is between pk = 19403 and 2pi−n = 19438 and is 19417.
The Ramanujan Prime Corollary is due to John Nicholson.
- Ramanujan, S. (1919), "A proof of Bertrand's postulate", Journal of the Indian Mathematical Society 11: 181–182
- Jonathan Sondow, "Ramanujan Prime", MathWorld.
- Sondow, J. (2009), "Ramanujan primes and Bertrand's postulate", Amer. Math. Monthly 116: 630–635, arXiv:0907.5232
- Laishram, S. (2010), "On a conjecture on Ramanujan primes", International Journal of Number Theory 6: 1869–1873.
- Sondow, J.; Nicholson, J.; Noe, T.D. (2011), "Ramanujan primes: bounds, runs, twins, and gaps", Journal of Integer Sequences 14: 11.6.2
- Amersi, N.; Beckwith, O.; Miller, S.J.; Ronan, R.; Sondow, J. (2011), Generalized Ramanujan primes, arXiv:1108.0475