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On twin primes associated with the Hawkins random sieve

Journal of Number Theory, 2006
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Journal of Number Theory 119 (2006) 284–296 www.elsevier.com/locate/jnt On twin primes associated with the Hawkins random sieve H.M. Bui , J.P. Keating School of Mathematics, University of Bristol, Bristol, BS8 1TW, UK Received 13 July 2005; revised 27 July 2005 Available online 4 January 2006 Communicated by J. Brian Conrey Abstract We establish an asymptotic formula for the number of k-difference twin primes associated with the Hawkins random sieve, which is a probabilistic model of the Eratosthenes sieve. The formula for k = 1 was obtained by M.C. Wunderlich [A probabilistic setting for prime number theory, Acta Arith. 26 (1974) 59–81]. We here extend this to k 2 and generalize it to all l -tuples of Hawkins primes. 2005 Published by Elsevier Inc. 1. Introduction The random sieve was introduced by Hawkins [4,5] as follows. Let S 1 ={2, 3, 4, 5,...}. Put P 1 = min S 1 . Every element of the set S 1 \{P 1 } is then sieved out, independently of the others, with probability 1/P 1 , and S 2 is the set of the surviving elements. In general, at the nth step, define P n = min S n . We then use 1/P n as the probability with which to delete the numbers in S n \{P n }. The set remaining is denoted by S n+1 . The Hawkins sieve is essentially a probabilistic analogue of the sieve of Eratosthenes. The sequences {P 1 ,P 2 ,...,P n ,...} of Hawkins primes mimic the primes in the sense that their statistical distribution is expected to be like that of the primes. The primes themselves correspond to one realization of the process. A great deal is known about the Hawkins primes. For instance, the analogues of the prime number theorem [5,6,9], Mertens’ theorem [6,9] and the Riemann hypothesis [7,8] are true with * Corresponding author. E-mail address: hm.bui@bristol.ac.uk (H.M. Bui). 0022-314X/$ – see front matter 2005 Published by Elsevier Inc. doi:10.1016/j.jnt.2005.11.015
H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 285 probability 1. We here concern ourselves with the density of k -difference Hawkins twin primes and its generalization to other l -tuples. Instead of a sequence of probability spaces, as considered by Hawkins, Wunderlich [9] sim- plified the process in a single probability space. Let X be the space of all sequences of integers greater than 1, i.e., X consists of all finite and infinite sequences. The class of all sets of those sequences is Ω . For α X, we denote by α n the set of elements of α which are less than n, i.e., α n = α ∩{2, 3, 4,...,n 1} and α n = α \ α n . Definition 1. An element E Ω is called an elementary set if there exists a sequence {a 1 ,a 2 ,...,a k }∈ X and an integer n>a k such that E consists of all the sequences α such that α n ={a 1 ,a 2 ,...,a k }. E is denoted by {a 1 ,a 2 ,...,a k ; n}, and if k = 0, E = {· ; n} is the set of all sequences whose elements are not less than n. The probability function is now defined recursively on the class of elementary sets. Definition 2. Define a non-negative real-valued function μ on the class of elementary sets as follows: (i) μ({· ; 2}) = 1, (ii) μ({a 1 ,...,a k ,n; n + 1}) = k i =1 (1 1 a i )μ({a 1 ,...,a k ; n}), (iii) μ({a 1 ,...,a k ; n + 1}) = (1 k i =1 (1 1 a i ))μ({a 1 ,...,a k ; n}). For any α X, the analogue of the k -difference twin prime counting function is defined as Π X,X+k (x ; α) = #{j x : j α and j + k α}. Wunderlich [9] showed that Π X,X+1 (x) x/(log x) 2 almost surely, which is an analogue of Hardy and Littlewood’s famous conjecture concerning the distribution of the twin primes [3]. The absence of the twin prime constant factor here is due to the drawback of the probabilistic setting of the random sieve that it contains little arithmetical information about the primes. Though the result is not unexpected, it is not easy to establish, as it is, for example, in Cramer’s model [1], where every number n is independently deleted with probability 1/ log n. In Section 2, we follow the lines of Wunderlich [9] and extend the result to k = 2, Theorem 1. Almost surely Π X,X+2 (x) x (log x) 2 . Theorem 1 requires rather more work than [9, Theorem 4], but the idea is similar and straight- forward. Nevertheless, it is clear from the proof for k = 2 that as k increases, the calculations will become extremely complicated, and the proof for the general case using Wunderlich’s method is likely to be extremely messy. In Section 3, we therefore develop a different approach and establish the following theorem.
Journal of Number Theory 119 (2006) 284–296 www.elsevier.com/locate/jnt On twin primes associated with the Hawkins random sieve H.M. Bui ∗ , J.P. Keating School of Mathematics, University of Bristol, Bristol, BS8 1TW, UK Received 13 July 2005; revised 27 July 2005 Available online 4 January 2006 Communicated by J. Brian Conrey Abstract We establish an asymptotic formula for the number of k-difference twin primes associated with the Hawkins random sieve, which is a probabilistic model of the Eratosthenes sieve. The formula for k = 1 was obtained by M.C. Wunderlich [A probabilistic setting for prime number theory, Acta Arith. 26 (1974) 59–81]. We here extend this to k  2 and generalize it to all l-tuples of Hawkins primes.  2005 Published by Elsevier Inc. 1. Introduction The random sieve was introduced by Hawkins [4,5] as follows. Let S1 = {2, 3, 4, 5, . . .}. Put P1 = min S1 . Every element of the set S1 \ {P1 } is then sieved out, independently of the others, with probability 1/P1 , and S2 is the set of the surviving elements. In general, at the nth step, define Pn = min Sn . We then use 1/Pn as the probability with which to delete the numbers in Sn \ {Pn }. The set remaining is denoted by Sn+1 . The Hawkins sieve is essentially a probabilistic analogue of the sieve of Eratosthenes. The sequences {P1 , P2 , . . . , Pn , . . .} of Hawkins primes mimic the primes in the sense that their statistical distribution is expected to be like that of the primes. The primes themselves correspond to one realization of the process. A great deal is known about the Hawkins primes. For instance, the analogues of the prime number theorem [5,6,9], Mertens’ theorem [6,9] and the Riemann hypothesis [7,8] are true with * Corresponding author. E-mail address: hm.bui@bristol.ac.uk (H.M. Bui). 0022-314X/$ – see front matter  2005 Published by Elsevier Inc. doi:10.1016/j.jnt.2005.11.015 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 285 probability 1. We here concern ourselves with the density of k-difference Hawkins twin primes and its generalization to other l-tuples. Instead of a sequence of probability spaces, as considered by Hawkins, Wunderlich [9] simplified the process in a single probability space. Let X be the space of all sequences of integers greater than 1, i.e., X consists of all finite and infinite sequences. The class of all sets of those sequences is Ω. For α ∈ X, we denote by αn the set of elements of α which are less than n, i.e., αn = α ∩ {2, 3, 4, . . . , n − 1} and α n = α \ αn . Definition 1. An element E ∈ Ω is called an elementary set if there exists a sequence {a1 , a2 , . . . , ak } ∈ X and an integer n > ak such that E consists of all the sequences α such that αn = {a1 , a2 , . . . , ak }. E is denoted by {a1 , a2 , . . . , ak ; n}, and if k = 0, E = {· ; n} is the set of all sequences whose elements are not less than n. The probability function is now defined recursively on the class of elementary sets. Definition 2. Define a non-negative real-valued function μ on the class of elementary sets as follows: (i) μ({· ; 2}) = 1,  (ii) μ({a1 , . . . , ak , n; n + 1}) = ki=1 (1 − a1i )μ({a1 , . . . , ak ; n}),  (iii) μ({a1 , . . . , ak ; n + 1}) = (1 − ki=1 (1 − a1i ))μ({a1 , . . . , ak ; n}). For any α ∈ X, the analogue of the k-difference twin prime counting function is defined as ΠX,X+k (x; α) = #{j  x: j ∈ α and j + k ∈ α}. Wunderlich [9] showed that ΠX,X+1 (x) ∼ x/(log x)2 almost surely, which is an analogue of Hardy and Littlewood’s famous conjecture concerning the distribution of the twin primes [3]. The absence of the twin prime constant factor here is due to the drawback of the probabilistic setting of the random sieve that it contains little arithmetical information about the primes. Though the result is not unexpected, it is not easy to establish, as it is, for example, in Cramer’s model [1], where every number n is independently deleted with probability 1/ log n. In Section 2, we follow the lines of Wunderlich [9] and extend the result to k = 2, Theorem 1. Almost surely ΠX,X+2 (x) ∼ x . (log x)2 Theorem 1 requires rather more work than [9, Theorem 4], but the idea is similar and straightforward. Nevertheless, it is clear from the proof for k = 2 that as k increases, the calculations will become extremely complicated, and the proof for the general case using Wunderlich’s method is likely to be extremely messy. In Section 3, we therefore develop a different approach and establish the following theorem. 286 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 Theorem 2. Almost surely, for any fixed integer k, as x → ∞, ΠX,X+k (x) ∼ x . (log x)2 As we note in Section 3, our approach extends straightforwardly to l-tuples of Hawkins primes to yield: Theorem 3. Let 0 < k1 < k2 < · · · < kl−1 and denote by ΠX,X+k1 ,...,X+kl−1 (x; α) the number of m  x such that the set {m, m + k1 , . . . , m + kl−1 } ⊂ α. Then as x → ∞, almost surely x . (log x)l ΠX,X+k1 ,...,X+kl−1 (x) ∼ An immediate corollary of this theorem is: Corollary 1. For any positive integers d, l, and l  2, almost surely, as x → ∞, ΠX,X+d,...,X+(l−1)d (x) ∼ x , (log x)l which is reminiscent of a recent theorem of Green and Tao [2] on the existence of arbitrarily long arithmetic progressions in the primes, proved using powerful techniques from analytic number theory, combinatorics and ergodic theory. 2. Proof of Theorem 1 We begin the proof by stating a lemma from [9]. Lemma 1. For r, s, t non-negative integers, r  t, define Mk = 1 + k−2  1 j =1 j . Then  s+1  n  c(t − r − 1, r)ns+1 ks n ns+1 c(1, r)ns+1 + ··· + +O = + r+1 t−1 Mkr (s + 1)Mnr Mnt Mn Mn k=2 =  s+1  t−r−1  ns+1 ns+1 n , c(j, r) + + O r r+j (s + 1)Mn Mnt (s + 1)Mn j =1 where c(j, r) = r(r + 1) · · · (r + j − 1)/(s + 1)j +1 . As in [9], we define ym (α) =  j <m, j ∈α   1 1− . j H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 287 Then P(m ∈ α) = ym (α), and if we let Cn be the set of all sequences containing n, μ(Cn ) = E(yn ) =  yn dμ. Wunderlich then obtained the asymptotic formula for the kth moment of yn , which is an analogue of Mertens’ theorem,    k 1 1 E ym = k + O . k+2 Mm Mm Some simple calculations give      1 1 3 1 2 P(m ∈ α, m + 2 ∈ α) = 1 − ym 1− ym (α) . (α) − m m+1 m Define the auxiliary function ΠX,X+2 (x; α) =  mx, m∈α m+2∈α  ym (α) −   −1 1 2 1 1− ym (α) . m+1 m In what follows, we write Π(x; α) for ΠX,X+2 (x; α), and if f : R → R, we define the usual difference operator  applied to f by f (m) := f (m + 1) − f (m). We have Π(m; α) =  ym+1 (α) − 0 1 m+2  1− 1 m+1 2 ym+1 (α) −1 if m + 1 ∈ α, m + 3 ∈ α, otherwise. (1) Hence    E Π(m + 1) − E Π(m) = 1 −  1 E(ym+1 ). m+1 Thus     n   1 1 1 1 1− E(ym+1 ) = +O 3 m+1 m Mm Mm m=3 m=2   n n n = + 2 +O . Mn Mn Mn3 n−1    1− E Π(n) = (2) We now wish to estimate the variance of Π(n). It is easy to see from (1) that   E Π 2 (m) = 2 1 −   1 E ym+1 Π(m) + O(1). m+1 (3) 288 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 i It is necessary to find another recursion for ym+1 (α)Π (m; α). We have i ym+2 (α)Π (m + 1; α) ⎧    1 1 1 i i 2 ⎪ 1 − m+1 ym+1 Π(m; α) + ym+1 (α) − m+2 1 − m+1 ym+1 (α) ⎪ ⎪ ⎪ ⎨ if m + 1 ∈ α, m + 3 ∈ α, =  i i 1 ⎪ 1 − m+1 ym+1 Π(m; α) if m + 1 ∈ α, m + 3 ∈ / α, ⎪ ⎪ ⎪ ⎩ i ym+1 Π(m; α) if m + 1 ∈ / α. −1 Since ⎧   2  1 1 1 3 ⎪ ⎨ P(m + 1 ∈ α, m + 3 ∈ α) = 1 − m+1 ym+1 (α) − m+2 1 − m+1 ym+1 (α) ,    1 1 1 3 2 P(m + 1 ∈ α, m + 3 ∈ / α) = ym+1 (α) − 1 − m+1 ym+1 (α) − m+2 1 − m+1 ym+1 (α) , ⎪ ⎩ P(m + 1 ∈ / α) = 1 − ym+1 (α), we easily obtain   i E ym+1 Π(m) = 1 − 1 m+1 i+1    i+1 − 1− 1− E ym+1 1 m+1 i   i+1 E ym+1 Π(m) . 4 Π (k)) = O(kE(y 4 )), we have Taking i = 3, summing from 1 to n − 1, and using E(yk+1 k+1  n   n       1  3 n 4 . E yn+1 Π(n) = O E ym = O =O 4 Mm Mn4 m=2 m=2 Letting i = 2 in (4),  2  2 Π(m + 1) − E ym+1 Π(m) E ym+2 3 2      3  3 1 1 E ym+1 Π (m) . = 1− E ym+1 − 1− 1− m+1 m+1 Summing from 1 to n − 1, we obtain  n   n    1  2 1 3  3 1− E yn+1 Π(n) = E ym + O 4 m Mm m=2 m=2   n   n   1 1 n n . = = + O + O 3 4 Mm Mm Mn3 Mn4 m=2 m=2 We are now ready to find E(ym+1 Π(m)). Letting i = 1 in (4),    E ym+2 Π (m + 1) − E ym+1 Π(m) = 1 − 1 m+1 2  2 E ym+1 −  2 1 E ym+1 Π(m) . m+1 (4) H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 289 So  n  n    1 2  2 1  2 1− E yn+1 Π(n) = E ym − E ym Π (m − 1) m m m=2 m=2  n    n n   1  n 1 1 n n = − +O = 2 + 3 +O . 2 3 4 Mm Mm Mm Mn Mn Mn4 m=2 m=2 m=2 Substituting this into (3), we have     n   2 m m m 1 + O(n) + 3 +O 2 1− E Π (n) = 2 4 m Mm Mm Mm m=2  2  2  2   2  n n n2 n n =2 +2 + +O +O 2Mn2 2Mn3 Mn4 2Mn3 Mn4  2 n2 2n2 n = 2 + 3 +O . Mn Mn Mn4 From (2) and (5) we deduce that ⎧ ⎨ E(Π(n)) = n Mn + Mn2 + O n  ⎩ Var(Π (n)) = O n24 . M  n Mn3 , n Theorem 2 in [9] then implies that Π(n) ∼ n . log n Now we define rm (α) = Then ΠX,X+2 (n; α) =   1 if m ∈ α, m + 2 ∈ α, 0 otherwise. rm (α) mn  rm (α) =  1 y (α) − m+1 1 − mn m  = am (α)bm (α), 1 m 2 (α) ym  ym (α) −    1 2 1 1− y (α) m+1 m m mn where am (α) = ym (α) − rm (α)  1 1 m+1 1 − m 2 (α) ym (5) 290 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 and   1 2 1 bm (α) = ym (α) − 1− y (α). m+1 m m Let A0 (α) = 0 and Am (α) = ΠX,X+2 (n; α) =  m j =1 aj (α). Using Abel summation, am (α)bm (α) = An (α)bn (α) −  m<n mn  Am (α) bm+1 (α) − bm (α) . Since An (α) = Π(n; α), An bn ∼ (n/ log n)(1/ log n) ∼ n/(log n)2 . The result follows if we can show that    n . Am |bm+1 − bm | = o (log n)2 m<n Firstly,  Am (α) = j m, j ∈α j +2∈α = 3 2 yj (α) −  j m, j ∈α j +2∈α 1 j +1 1 , yj (α) 1  1− 1 j yj2 (α)   j m, j ∈α j +2∈α 1 yj (α) − 31 yj (α) which is O(m/ log m) from [9, Theorem 4]. Secondly,   bm+1 (α) − bm (α)           1 1 2 1 1 2 =  ym+1 (α) − 1− ym+1 1− ym (α) . (α) − ym (α) − m+2 m+1 m+1 m Since ym+1 (α) =  1 − m1 ym (α) = ym (α) m−1 m ym (α) if m ∈ α, otherwise, we obtain So   bm+1 (α) − bm (α) =   1 − ym (α) + 3(m−1) y 2 (α) m m(m+1)(m+2) m   m−2  y 2 (α) m(m+1)(m+2) m   bm+1 (α) − bm (α)   1 m ym (α) 1 2 m(m+1) ym (α) if m ∈ α, otherwise. if m ∈ α, otherwise. 291 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 Hence  m<n      Am (α)bm+1 (α) − bm (α) Am (α)bm+1 (α) − bm (α) = m<n m∈α +  m<n m∈α /    Am (α)bm+1 (α) − bm (α)  1  1 2 Am (α)ym (α) + Am (α)ym (α). m m(m + 1) m<n m<n m∈α m∈α / Thus  m<n   Am (α)bm+1 (α) − bm (α) = O =O    1 (log m)2  +O  1 (log m)2  + O(1), m<n m∈α m<n m∈α   m<n m∈α / 1 m(log m)3  which is easily seen to be O(n/(log n)3 ) from the analogue of prime number theorem for Hawkins random sieve. The result follows. 3. Proof of Theorems 2 and 3 In this section, we take m[i1 , i2 , . . . , il ] ∈ α, where i1 < i2 < · · · < il , to mean m + {i1 , i2 , . . . , il } ⊂ α, and m + h ∈ / α for all h ∈ [i1 , il ] \ {i1 , i2 , . . . , il }. Lemma 2. Given a non-negative integer l and 0 = i0 < i1 < i2 < · · · < il < il+1 = k, define T[0,i1 ,i2 ,...,il ,k] (n) := mn  1. m[0,i1 ,i2 ,...,il ,k]∈α Then T[0,i1 ,i2 ,...,il ,k] (n) ∼ n/(log n)l+2 almost surely. Proof. We simply write T (n) for T[0,i1 ,i2 ,...,il ,k] (n). Let An be the event n ∈ α and Bn be the complement of An , i.e., Bn = Acn . We then have  Pm = P (m + 1)[0, i1 , i2 , . . . , il , k] ∈ α = P(Am+k+1 Bm+k . . . Bm+2+il Am+1+il . . . Bm+2 Am+1 ). 292 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 By the chain rule Pm = P(Am+1 ) × P(Bm+2 |Am+1 ) . . . P(Bm+i1 |Bm+i1 −1 . . . Bm+2 Am+1 ) × P(Am+1+i1 |Bm+i1 . . . Bm+2 Am+1 ) × ··· × P(Am+k+1 |Bm+k . . . Bm+2+il Am+1+il . . . Bm+2 Am+1 ) = ym+1 i1 −1  1 ym+1 m+1    1 ym+1 × 1− m+1 × ···       1 1 1 1− ··· 1 − ym+1 , × 1− m + 1 + il m + 1 + il−1 m+1   × 1− 1− or, in short, l   1− Pm = j =0 1 m + 1 + ij l+1−j l+2 ym+1 l  j =0  l−j   1− 1− h=0 il+1−j −il−j −1  1 ym+1 . m + 1 + ih Since l  j =0  l−j   1− 1− h=0 il+1−j −il−j −1  1 ym+1 m + 1 + ih   2 ym+1 + O ym+1 (il+1−j − il−j − 1)ym+1 + O =1− m j =0    2 ym+1 = 1 − (k − l − 1)ym+1 + O + O ym+1 , m  l  we have l+2 Pm = ym+1 − (k − l l+3 − 1)ym+1 +O  l+3 ym+1 m   l+4 + O ym+1 . From the definition of T (m), T (m + 1) − T (m) =  1 if (m + 1)[0, i1 , i2 , . . . , il , k] ∈ α, 0 otherwise. H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 293 Hence  l+3  E(ym+1 )  l+3    l+2   l+4 E T (m + 1) − E T (m) = E ym+1 − (k − l − 1)E ym+1 +O + O E ym+1 m     1 1 k−l−1 1 +O = l+2 − +O l+3 l+4 l+3 Mm+1 Mm+1 Mm+1 mMm+1   k−l−1 1 1 . = l+2 − + O l+3 l+4 Mm+1 Mm+1 Mm+1 Summing from 1 to n − 1 yields  E T (n) = = n Mnl+2 n Mnl+2 (l + 2)n  (k − l − 1)n n +O Mnl+3 Mnl+4   (k − 2l − 3)n n . − +O l+3 Mn Mnl+4 + Mnl+3 −  (6) The next step is to estimate the variance of T (n). As in the case of the previous theorem, we i need to establish a recursion for ym+1 T (m). For this we have ⎧ i y T (m) ⎪ ⎪ ⎨  m+1 1 i i i 1 − m+1 ym+1 (T (m) + 1) ym+2 T (m + 1) = ⎪ ⎪ ⎩ i 1 i 1 − m+1 ym+1 T (m) if m + 1 ∈ / α, if (m + 1)[0, i1 , i2 , . . . , il , k] ∈ α, otherwise. Since P(m + 1 ∈ / α) = 1 − ym+1 , P((m + 1)[0, i1 , i2 , . . . , il , k] ∈ α) = Pm , we deduce that   i  i T (m + 1) = E ym+1 T (m)(1 − ym+1 ) + 1 − E ym+2  + 1− 1 m+1 i 1 m+1 i  i  E ym+1 T (m) + 1 Pm  i E ym+1 T (m)(ym+1 − Pm ) , or, equivalently,   i E ym+1 T (m) = 1 − 1 m+1 i    i E ym+1 Pm − 1 − 1 − 1 m+1 i  Letting i = l + 4, and recalling that  l+4 l+2 l+3 Pm = ym+1 − (k − l − 1)ym+1 + O ym+1 ,  i+1 E ym+1 T (m) . (7) 294 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 we obtain  l+4   2l+6 E ym+1 T (m) = O E ym+1 =O  1 2l+6 Mm+1  . So  n   l+4 E yn+1 T (n) = O m=2 1 2l+6 Mm  =O  n Mn2l+6  . Letting i = l + 3 in (7), and summing from 1 to n − 1, we have n   l+3 T (n) = E yn+1 m=2 1 +O 2l+5 Mm  n  m=2 1 2l+6 Mm  = n Mn2l+5 +O   . 2l+6 n Mn Finally, substituting i = l + 2 in (7),   l+2 E ym+1 T (m) = 1 − = 1 m+1 1 2l+4 Mm+1 l+2 − (k + 1)    l+2 E ym+1 Pm − 1 − 1 − 1 2l+5 Mm+1 +O  1 2l+6 Mm+1  1 m+1 l+2   l+3 E ym+1 T (m) . Thus n   l+2 E yn+1 T (n) = 1 2l+4 Mm m=2 = = n Mn2l+4 n Mn2l+4 − (k + 1) + (2l + 4) n  1 2l+5 Mm m=2 n Mn2l+5 − (k − 2l − 3) +O − (k + 1) n Mn2l+5  n n  m=2 1 2l+6 Mm   n +O Mn2l+5 Mn2l+6   n . +O Mn2l+6  Now, back to the variance of T (n), T 2 (m + 1) = T 2 (m) + 2T (m) + 1 if (m + 1)[0, i1 , i2 , . . . , il , k] ∈ α, T 2 (m) otherwise. Therefore    E T 2 (m + 1) = E T 2 (m)(1 − Pm ) + E T 2 (m) + 2T (m) + 1 Pm   = E T 2 (m) + 2E T (m)Pm + E(Pm ). (8) 295 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 And hence   l+2  l+3 E T 2 (m) = 2E ym+1 T (m) − 2(k − l − 1)E ym+1 T (m)   l+4   l+2 + O E ym+1 T (m) + O E ym+1 . From (8), we obtain   m m − 2(k − l − 1) + O 2l+4 2l+5 2l+5 2l+6 Mm Mm Mm Mm   2m 2(2k − 3l − 4)m m . = 2l+4 − +O 2l+5 2l+6 Mm Mm Mm  E T 2 (m) = 2  m  m − (k − 2l − 3) So n   E T 2 (n) = m=2 =  2m 2l+4 Mm n2 − (2k − 3l − 4) + (l + 2)n2  n  m=2 2m 2l+5 Mm − (2k − 3l − 4) +O  n Mnl+2 ⎩ Var(T (n)) = O  − (k−2l−3)n Mnl+3 n2 Mn2l+6 +O .  n2 n Mnl+4 m  2l+6 Mm  2  n +O Mn2l+6 m=2 Mn2l+4 Mn2l+5 Mn2l+5   (2k − 4l − 6)n2 n2 n2 . = 2l+4 − +O Mn Mn2l+5 Mn2l+6 Combining (6) with (9), we have ⎧ ⎨ E(T (n)) = n  (9) , Theorem 2 in [9] again yields T (n) ∼ n/(log n)l+2 as asserted. The proof of Theorem 2 now follows immediately from Lemma 2 by noting that ΠX,X+k (x) = k−1   T[0,i1 ,i2 ,...,il ,k] (x) l=0 0<i1 <i2 <···<il <k = T[0,k] (x) + k−1   T[0,i1 ,i2 ,...,il ,k] (x) l=1 0<i1 <i2 <···<il <k Similarly for Theorem 3,  = 1 + o(1)   x x . + O (log x)2 (log x)3  x (log x)l+1    x x . + O = 1 + o(1) (log x)l (log x)l+1 ΠX,X+k1 ,...,X+kl−1 (x) = T[0,k1 ,k2 ,...,kl−1 ] (x) + O  ✷ 296 H.M. Bui, J.P. Keating / Journal of Number Theory 119 (2006) 284–296 Acknowledgment J.P.K. is supported by an EPSRC Senior Research Fellowship. References [1] H. Cramer, On the order of magnitude of the differences between consecutive prime numbers, Acta Arith. 2 (1937) 23–28. [2] B. Green, T. Tao, The primes contain arbitrarily long arithmetic progressions, math.NT/0404188. [3] G.H. Hardy, J.E. Littlewood, Some problems in “Partitio Numerorum” III: On the expression of a number as a sum of primes, Acta Math. 44 (1923) 1–70. [4] D. Hawkins, The random sieve, Math. Mag. 31 (1957/1958) 1–3. [5] D. Hawkins, Random sieves II, J. Number Theory 6 (1974) 192–200. [6] C.C. Heyde, On asymptotic behaviour for the Hawkins random sieve, Proc. Amer. Math. Soc. 56 (1976) 277–280. [7] C.C. Heyde, A log log improvement to the Riemann hypothesis for the Hawkins random sieve, Ann. Probab. 6 (5) (1978) 870–875. [8] W. Neudecker, D. Williams, The “Riemann hypothesis” for the Hawkins random sieve, Compos. Math. 29 (1974) 197–200. [9] M.C. Wunderlich, A probabilistic setting for prime number theory, Acta Arith. 26 (1974) 59–81. Further reading [10] W. Neudecker, On twin “primes” and gaps between successive “primes” for the Hawkins random sieve, Math. Proc. Cambridge Philos. Soc. 77 (1975) 365–367. [11] P. Ribenboim, The Book of Prime Number Records, second ed., Springer, New York, 1989. [12] M.C. Wunderlich, The prime number theorem for the random sequences, J. Number Theory 8 (1976) 369–371.
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