

A104272


Ramanujan primes R_n: a(n) is the smallest number such that if x >= a(n), then pi(x)  pi(x/2) >= n, where pi(x) is the number of primes <= x.


148



2, 11, 17, 29, 41, 47, 59, 67, 71, 97, 101, 107, 127, 149, 151, 167, 179, 181, 227, 229, 233, 239, 241, 263, 269, 281, 307, 311, 347, 349, 367, 373, 401, 409, 419, 431, 433, 439, 461, 487, 491, 503, 569, 571, 587, 593, 599, 601, 607, 641, 643, 647, 653, 659
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OFFSET

1,1


COMMENTS

Referring to his proof of Bertrand's postulate, Ramanujan states a generalization: "From this we easily deduce that pi(x)  pi(x/2) >= 1, 2, 3, 4, 5, ..., if x >= 2, 11, 17, 29, 41, ..., respectively." Since the a(n) are prime (by their minimality), I call them "Ramanujan primes."
See the additional references and links mentioned in A143227.
2n log 2n < a(n) < 4n log 4n for n >= 1, and prime(2n) < a(n) < prime(4n) if n > 1. Also, a(n) ~ prime(2n) as n > infinity.
Shanta Laishram has proved that a(n) < prime(3n) for all n >= 1.
a(n)  3n log 3n is sometimes positive, but negative with increasing frequency as n grows since a(n) ~ 2n log 2n. There should be a constant m such that for n >= m we have a(n) < 3n log 3n.
A good approximation to a(n) = R_n for n in [1..1000] is A162996(n) = Round(k*n * (log(k*n)+1)), with k = 2.216 determined empirically from the first 1000 Ramanujan primes, which approximates the {k*n}th prime number which in turn approximates the nth Ramanujan prime and where Abs(A162996(n)  R_n) < 2 * Sqrt(A162996(n)) for n in [1..1000]. Since R_n ~ prime(2n) ~ 2n * (log(2n)+1) ~ 2n * log(2n), while A162996(n) ~ prime(k*n) ~ k*n * (log(k*n)+1) ~ k*n * log(k*n), A162996(n) / R_n ~ k/2 = 2.216/2 = 1.108 which implies an asymptotic overestimate of about 10% (a better approximation would need k to depend on n and be asymptotic to 2.)  Daniel Forgues, Jul 29 2009
Let p_n be the nth prime. If p_n>=3 is in the sequence, then all integers (p_n+1)/2, (p_n+3)/2, ..., (p_(n+1)1)/2 are composite numbers.  Vladimir Shevelev, Aug 12 2009
Denote by q(n) the prime which is the nearest from the right to a(n)/2. Then there exists a prime between a(n) and 2q(n). Converse, generally speaking, is not true, i.e., there exist primes outside the sequence, but possess such property (e.g., 109).  Vladimir Shevelev, Aug 14 2009
The Mathematica program FasterRamanujanPrimeList uses Laishram's result that a(n) < prime(3n).
About 46% of primes < 19000 are Ramanujan primes. About 78% of the lesser of twin primes < 19000 are Ramanujan primes.
About 15% of primes < 19000 are the lesser of twin primes. About 26% of Ramanujan primes < 19000 are the lesser of twin primes.
A reason for the jumps is in Section 7 of "Ramanujan primes and Bertrand's postulate" and in Section 4 of "Ramanujan Primes: Bounds, Runs, Twins, and Gaps". (See the arXiv link for a corrected version of Table 1.)
See Shapiro 2008 for an exposition of Ramanujan's proof of his generalization of Bertrand's postulate. (End)
The (10^n)th R prime: 2, 97, 1439, 19403, 242057, 2916539, 34072993, 389433437, ....  Robert G. Wilson v, May 07 2011, updated Aug 02 2012
The number of R primes < 10^n: 1, 10, 72, 559, 4459, 36960, 316066, 2760321, ....  Robert G. Wilson v, Aug 02 2012
a(n) = R_n = R_{0.5,n} in "Generalized Ramanujan Primes."
Or the largest prime x such that the number of primes in (x/2,x] equals n. This equivalent definition underlines an important analogy between Ramanujan and Labos primes (cf. A080359).  Vladimir Shevelev, Apr 29 2012
The questions on R_n  prime(2n) in A233739 have been answered by Christian Axler in "On generalized Ramanujan primes".  Jonathan Sondow, Feb 13 2014
Srinivasan's Lemma (2014): prime(kn) < prime(k)/2 if R_n = prime(k) and n > 1. Proof: By the minimality of R_n, the interval (prime(k)/2,prime(k)] contains exactly n primes and so prime(kn) < prime(k)/2.  Jonathan Sondow, May 10 2014
For some n and k, we see that A168421(k) = a(n) so as to form a chain of primes similar to a Cunningham chain. For example (and the first example), A168421(2) = 7, links a(2) = 11 = A168421(3), links a(3) = 17 = A168421(4), links a(4) = 29 = A168421(6), links a(6) = 47. Note that the links do not have to be of a form like q = 2*p+1 or q = 2*p1.  John W. Nicholson, Feb 22 2015
Extending Sondow's 2010 comments: About 48% of primes < 10^9 are Ramanujan primes. About 76% of the lesser of twin primes < 10^9 are Ramanujan primes.  Dana Jacobsen, Sep 06 2015
Sondow, Nicholson, and Noe's 2011 conjecture that pi(R_{m*n}) <= m*pi(R_n) for m >= 1 and n >= N_m (see A190413, A190414) was proved for n > 10^300 by Shichun Yang and Alain Togbé in 2015.  Jonathan Sondow, Dec 01 2015
Berliner, Dean, Hook, Marr, Mbirika, and McBee (2016) prove in Theorem 18 that the graph K_{m,n} is prime for n >= R_{m1}m; see A291465.  Jonathan Sondow, May 21 2017
Okhotin (2012) uses Ramanujan primes to prove Lemma 8 in "Unambiguous finite automata over a unary alphabet."  Jonathan Sondow, May 30 2017
Sepulcre and Vidal (2016) apply Ramanujan primes in Remark 9 of "On the nonisolation of the real projections of the zeros of exponential polynomials."  Jonathan Sondow, May 30 2017


REFERENCES

Srinivasa Ramanujan, Collected Papers of Srinivasa Ramanujan (Ed. G. H. Hardy, S. Aiyar, P. Venkatesvara and B. M. Wilson), Amer. Math. Soc., Providence, 2000, pp. 208209.
Harold N. Shapiro, Ramanujan's idea, Section 9.3B in Introduction to the Theory of Numbers, Dover, 2008.


LINKS

N. Amersi, O. Beckwith, S. J. Miller, R. Ronan, J. Sondow, Generalized Ramanujan primes, Combinatorial and Additive Number Theory, Springer Proc. in Math. & Stat., CANT 2011 and 2012, Vol. 101 (2014), 113
Adam H. Berliner, N. Dean, J. Hook, A. Marr, A. Mbirika, C. McBee, Coprime and prime labelings of graphs, arXiv preprint arXiv:1604.07698 [math.CO], 2016; Journal of Integer Sequences, Vol. 19 (2016), #16.5.8.


FORMULA

a(n) = 1 + max{k: pi(k)  pi(k/2) = n  1}.
a(n) = max{prime p: pi(p)  pi(p/2) = n} (see Shevelev 2012).  Jonathan Sondow, Mar 23 2016


EXAMPLE

a(1) = 2 is Bertrand's postulate: pi(x)  pi(x/2) >= 1 for all x >= 2.
a(2) = 11 because a(2) < 8 log 8 < 17 and pi(n)  pi(n/2) > 1 for n = 16, 15, ..., 11 but pi(10)  pi(5) = 1.
Consider a(9)=71. Then the nearest prime > 71/2 is 37, and between a(9) and 2*37, that is, between 71 and 74, there exists a prime (73).  Vladimir Shevelev, Aug 14 2009 [corrected by Jonathan Sondow, Jun 17 2013]


MAPLE

local R;
if n = 1 then
return 2;
end if;
R := ithprime(3*n1) ; # upper limit Laishram's thrm Thrm 3 arXiv:1105.2249
while true do
if A056171(R) = n then # Defn. 1. of Shevelev JIS 14 (2012) 12.1.1
return R ;
end if;
R := prevprime(R) ;
end do:
end proc:


MATHEMATICA

(RamanujanPrimeList[n_] := With[{T=Table[{k, PrimePi[k]PrimePi[k/2]}, {k, Ceiling[N[4*n*Log[4*n]]]}]}, Table[1+First[Last[Select[T, Last[ # ]==i1&]]], {i, 1, n}]]; RamanujanPrimeList[54]) (* Jonathan Sondow, Aug 15 2009 *)
(FasterRamanujanPrimeList[n_] := With[{T=Table[{k, PrimePi[k]PrimePi[k/2]}, {k, Prime[3*n]}]}, Table[1+First[Last[Select[T, Last[ # ]==i1&]]], {i, 1, n}]]; FasterRamanujanPrimeList[54])
nn=1000; R=Table[0, {nn}]; s=0; Do[If[PrimeQ[k], s++]; If[PrimeQ[k/2], s]; If[s<nn, R[[s+1]]=k], {k, Prime[3*nn]}]; R=R+1 (* T. D. Noe, Nov 15 2010 *)


PROG

(Perl) use ntheory ":all"; my $r = ramanujan_primes(1000); say "[@$r]"; # Dana Jacobsen, Sep 06 2015
(PARI) ramanujan_prime_list(n) = {my(L=vector(n), s=0, k=1); for(k=1, prime(3*n)1, if(isprime(k), s++); if(k%2==0 && isprime(k/2), s); if(s<n, L[s+1] = k+1)); L} \\ Satish Bysany, Mar 02 2017


CROSSREFS

Cf. A000720, A007053, A014085, A060715, A084139, A084140, A143223, A143224, A143225, A143226, A143227, A080360, A080359, A164368, A164288, A164554, A164333, A164294, A164371, A190303.
Cf. A162996 (Round(kn * (log(kn)+1)), with k = 2.216 as an approximation of R_n = nth Ramanujan Prime.
Cf. A163160 (Round(kn * (log(kn)+1))  R_n, where k = 2.216 and R_n = nth Ramanujan prime).
Cf. A178127 (Lesser of twin Ramanujan primes), A178128 (Lesser of twin primes if it is a Ramanujan prime).
Cf. A181671 (number of Ramanujan primes less than 10^n).
Cf. A190124 (constant of summation: 1/a(n)^2).
Cf. A190413, A190414, A212493, A212541, A233739, A233822, A277718, A277719, A164952, A290394, A291465.
Not to be confused with the Ramanujan numbers or Ramanujan tau function, A000594.


KEYWORD

nonn,nice


AUTHOR



STATUS

approved



