Generalized Ramanujan Primes

In 1845, Bertrand conjectured that for all integers $x\ge 2$, there exists at least one prime in $(x\mathrm{/}2,x]$. This was proved by Chebyshev in 1860, and then generalized by Ramanujan in 1919. He showed that for any $n\ge 1$, there is a (smallest) prime $R_n$ such that $\pi (x)-\pi (x\mathrm{/}2)\ge n$ for all $x\ge R_n$. In 2009 Sondow called $R_n$ the $n$th Ramanujan prime and proved the asymptotic behavior $R_n\sim p_{2n}$ (where $p_m$ is the $m$th prime). In the present paper, we generalize the interval of interest by introducing a parameter $c \in (0,1)$anddefiningthe$n$th$c$-Ramanujanprimeasthesmallestinteger$R_{c,n}$suchthatforall$x\ge R_{c,n}$,thereareatleast$n$primesin$(cx,x]$. Using consequences of strengthened versions of the Prime Number Theorem, we prove that $R_{c,n}$ exists for all $n$ and all $c$, that $R_{c,n} \sim p_{\frac{n}{1-c}}$as$n\to\infty$, and that the fraction of primes which are $c$-Ramanujan converges to $1-c$. We then study finer questions related to their distribution among the primes, and see that the $c$-Ramanujan primes display striking behavior, deviating significantly from a probabilistic model based on biased coin flipping; this was first observed by Sondow, Nicholson, and Noe in the case $c=1\mathrm{/}2$. This model is related to the Cramer model, which correctly predicts many properties of primes on large scales, but has been shown to fail in some instances on smaller scales.