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A068828
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Geometrically weak primes: primes that are smaller than the geometric mean of their neighbors (2 is included by convention).
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4
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2, 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
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Or, bad primes (version 1): primes not in A046869. - Jonathan Vos Post (jvospost3(AT)gmail.com), Aug 20 2007
The idea can be extended by defining a geometrically weak prime of order k to be a prime which is less than the geometric mean of r neighbors on both sides for all r = 1 to k and not true for r = k+1. A similar extension could be defined for the sequence A051635.
It is easy to show that, except for the twin prime pair (3,5), the larger prime of every twin prime pair is in this sequence. The smaller prime of the pair is always in A046869. - T. D. Noe, Feb 19 2008
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..10000
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FORMULA
| prime(k)^2 <= prime(k-1)*prime(k+1).
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EXAMPLE
| 23 belongs to this sequence as 23^2 = 529 < 19*29= 551.
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MATHEMATICA
| Join[{2}, Prime[Select[Range[2, 120], Prime[ # ]^2 <= Prime[ # - 1]*Prime[ # + 1]&]]] - Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Aug 21 2007
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CROSSREFS
| Cf. A051634, A051635, A006562, A000040, A046869.
Sequence in context: A019411 A105792 A130903 * A100764 A076974 A051484
Adjacent sequences: A068825 A068826 A068827 * A068829 A068830 A068831
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KEYWORD
| easy,nonn
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AUTHOR
| Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Mar 08 2002
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EXTENSIONS
| Corrected and extended by Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Aug 21 2007
Edited by N. J. A. Sloane (njas(AT)research.att.com), Feb 19 2008
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