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 A071538 Number of twin prime pairs (p, p+2) with p <= n. 17
 0, 0, 1, 1, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 5, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8, 8 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS The convention is followed that a twin prime is <= n if its smaller member is <= n. Except for (3, 5), every pair of twin primes is congruent (-1, +1) (mod 6). - Daniel Forgues, Aug 05 2009 This function is sometimes known as pi_2(n). If this name is used, there is no obvious generalization for pi_k(n) for k > 2. - Franklin T. Adams-Watters, Jun 01 2014 REFERENCES S. Lang, The Beauty of Doing Mathematics, pp. 12-15; 21-22, Springer-Verlag NY 1985. LINKS Daniel Forgues, Table of n, a(n) for n = 1..99998 Thomas R. Nicely, Some Results of Computational Research in Prime Numbers. Eric Weisstein's World of Mathematics, Twin Primes. EXAMPLE a(30) = 5, since (29,31) is included along with (3,5), (5,7), (11,13) and (17,19). MATHEMATICA primePi2[1] = 0; primePi2[n_] := primePi2[n] = primePi2[n - 1] + Boole[PrimeQ[n] && PrimeQ[n + 2]]; Table[primePi2[n], {n, 100}] (* T. D. Noe, May 23 2013 *) PROG (PARI) A071538(n) = local(s=0, L=0); forprime(p=3, n+2, L==p-2 & s++; L=p); s /* For n > primelimit, one may use: */ A071538(n) = { local(s=isprime(2+n=precprime(n))&n, L); while( n=precprime(L=n-2), L==n & s++); s } /* The following gives a reasonably good estimate for small and for large values of n (cf. A007508): */ A071538est(n) = 1.320323631693739*intnum(t=2, n+1/n, 1/log(t)^2)-log(n) /* (The constant 1.320... is A114907.) */ \\ M. F. Hasler, Dec 10 2008 CROSSREFS Cf. A007508, A033843, A001359, A006512. Sequence in context: A098429 A132090 A235043 * A321017 A138194 A133876 Adjacent sequences:  A071535 A071536 A071537 * A071539 A071540 A071541 KEYWORD nonn AUTHOR Reinhard Zumkeller, May 30 2002 EXTENSIONS Definition edited by Daniel Forgues, Jul 29 2009 STATUS approved

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Last modified April 18 02:38 EDT 2021. Contains 343072 sequences. (Running on oeis4.)