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A051635
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Weak primes: prime(n) < (prime(n-1) + prime(n+1))/2.
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48
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3, 7, 13, 19, 23, 31, 43, 47, 61, 73, 83, 89, 103, 109, 113, 131, 139, 151, 167, 181, 193, 199, 229, 233, 241, 271, 283, 293, 313, 317, 337, 349, 353, 359, 383, 389, 401, 409, 421, 433, 443, 449, 463, 467, 491, 503, 509, 523, 547, 571, 577, 601, 619, 643, 647
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OFFSET
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1,1
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COMMENTS
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Primes prime(n) such that prime(n)-prime(n-1) < prime(n+1)-prime(n). [Juri-Stepan Gerasimov, Jan 01 2011].
a(n) < A051634(n). a(n) ~ 2*prime(n). [Thomas Ordowski, Jul 25 2012]
Erdős called a weak prime an "early prime." He conjectured that there are infinitely many consecutive pairs of early primes, and offered $100 for a proof and $25000 for a disproof (Kuperberg 1992). See A229832 for a stronger conjecture. - Jonathan Sondow, Oct 13 2013
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REFERENCES
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A. Murthy, Smarandache Notions Journal, Vol. 11 N. 1-2-3 Spring 2000
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LINKS
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T. D. Noe, Table of n, a(n) for n = 1..1000
Greg Kuperberg, The Erdos kitty: At least $9050 in prizes!, Newsgroups: rec.puzzles, sci.math, 1992. [Broken link]
Greg Kuperberg, The Erdos kitty: At least $9050 in prizes!, Newsgroups: rec.puzzles, sci.math, 1992. [Cached copy]
Wikipedia, Weak prime
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FORMULA
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a(1) = A229832(1). - Jonathan Sondow, Oct 13 2013
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EXAMPLE
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7 belongs to the sequence because 7 < (5+11)/2.
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MATHEMATICA
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Transpose[Select[Partition[Prime[Range[10^2]], 3, 1], #[[2]]<(#[[1]]+#[[3]])/2 &]][[2]] (* Vladimir Joseph Stephan Orlovsky, May 01 2008 *)
p=Prime[Range[200]]; p[[Flatten[1+Position[Sign[Differences[p, 2]], 1]]]]
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PROG
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(PARI) p=2; q=3; forprime(r=5, 1e3, if(2*q<p+r, print1(q", ")); p=q; q=r) \\ Charles R Greathouse IV, Jul 25 2011
(Haskell)
a051635 n = a051635_list !! (n-1)
a051635_list = g a000040_list where
g (p:qs@(q:r:ps)) = if 2 * q < (p + r) then q : g qs else g qs
-- Reinhard Zumkeller, May 09 2013
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CROSSREFS
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Cf. A006562, A051634, A229832.
Subsequence of A178943; A225495 (multiplicative closure).
Sequence in context: A105399 A133261 A113911 * A136009 A216098 A310262
Adjacent sequences: A051632 A051633 A051634 * A051636 A051637 A051638
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KEYWORD
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nice,nonn,easy
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AUTHOR
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Felice Russo, Nov 15 1999
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EXTENSIONS
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More terms from James A. Sellers
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STATUS
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approved
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