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 A000230 a(0)=2; for n>=1, a(n) = smallest prime p such that there is a gap of exactly 2n between p and next prime, or -1 if no such prime exists. (Formerly M2685 N1075) 91
 2, 3, 7, 23, 89, 139, 199, 113, 1831, 523, 887, 1129, 1669, 2477, 2971, 4297, 5591, 1327, 9551, 30593, 19333, 16141, 15683, 81463, 28229, 31907, 19609, 35617, 82073, 44293, 43331, 34061, 89689, 162143, 134513, 173359, 31397, 404597, 212701, 188029, 542603, 265621, 461717, 155921, 544279, 404851, 927869, 1100977, 360653, 604073 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS p + 1 = A045881(n) starts the smallest run of exactly 2n-1 successive composite numbers. - Lekraj Beedassy, Apr 23 2010 REFERENCES N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS N. J. A. Sloane, Table of n, a(n) for n = 0..673 (from Nicely's website) A. Booker, The Nth Prime Page H. Bottomley, Prime number calculator L. J. Lander and T. R. Parkin, On the first appearance of prime differences, Math. Comp., 21 (1967), 483-488. T. R. Nicely, List of prime gaps Tomás Oliveira e Silva, Gaps between consecutive primes J. Young and A. Potler, First occurrence prime gaps, Math. Comp., 52 (1989), 221-224. FORMULA a(n) = A000040(A038664(n)). - Lekraj Beedassy, Sep 09 2006 EXAMPLE The following table, based on a very much larger table in the web page of Tomás Oliveira e Silva (see link) shows, for each gap g, P(g) = the smallest prime such that P(g)+g is the smallest prime number larger than P(g); * marks a record-holder: g is a record-holder if P(g') > P(g) for all (even) g' > g, i.e., if all prime gaps are smaller than g for all primes smaller than P(g); P(g) is a record-holder if P(g') < P(g) for all (even) g' < g. This table gives rise to many sequences: P(g) is A000230, the present sequence; P(g)* is A133430; the positions of the *'s in the P(g) column give A100180, A133430; g* is A005250; P(g*) is A002386; etc. ----- g P(g) ----- 1* 2* 2* 3* 4* 7* 6* 23* 8* 89* 10 139* 12 199* 14* 113 16 1831* 18* 523 20* 887 22* 1129 24 1669 26 2477* 28 2971* 30 4297* 32 5591* 34* 1327 36* 9551* ........ The first time a gap of 4 occurs between primes is between 7 and 11, so a(2)=7 and A001632(2)=11. MATHEMATICA Join[{2}, With[{pr = Partition[Prime[Range], 2, 1]}, Transpose[ Flatten[ Table[Select[pr, #[] - #[] == 2n &, 1], {n, 50}], 1]][]]] (* Harvey P. Dale, Apr 20 2012 *) PROG (PARI) a(n)=my(p=2); forprime(q=3, , if(q-p==2*n, return(p)); p=q) \\ Charles R Greathouse IV, Nov 20 2012 (Perl) use ntheory ":all"; my(\$l, \$i, @g)=(2, 0); forprimes { \$g[(\$_-\$l) >> 1] //= \$l;  while (defined \$g[\$i]) { print "\$i \$g[\$i]\n"; \$i++; }  \$l=\$_; } 1e10; # Dana Jacobsen, Mar 29 2019 CROSSREFS A001632(n) = 2n + a(n) = nextprime(a(n)). Cf. A001223, A002386, A005250, A045881, A038664. Cf. A100964 (least prime number that begins a prime gap of at least 2n). Cf. also A133429 (records), A133430, A100180, A226657, A229021, A229028, A229030, A229033, A229034. Sequence in context: A163834 A335366 A002386 * A256454 A133429 A087770 Adjacent sequences:  A000227 A000228 A000229 * A000231 A000232 A000233 KEYWORD nonn,nice AUTHOR EXTENSIONS a(29)-a(37) from Jud McCranie, Dec 11 1999 a(38)-a(49) from Robert A. Stump (bee_ess107(AT)yahoo.com), Jan 11 2002 "or -1 if ..." added to definition at the suggestion of Alexander Wajnberg by N. J. A. Sloane, Feb 02 2020 STATUS approved

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Last modified September 18 20:19 EDT 2020. Contains 337173 sequences. (Running on oeis4.)