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Search: id:A088121
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| A088121 |
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Smallest prime obtained as a sum of n terms of a geometric progression + the common ratio, or 0 if no such terms exists. Smallest prime of the form (a +ar +ar^2 + ar^3 +... ) + r. |
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+0
2
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| 3, 5, 23, 17, 157, 191, 383, 257, 2557, 9209, 6143, 20477, 73721, 147449, 360439, 65537, 655357, 786431, 11010029, 5242877, 31457267, 71303153, 276824033, 150994937, 301989881, 469762043, 671088637, 4026531827, 2684354557
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OFFSET
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1,1
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COMMENT
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The ratio is required to be > 1.
For a(5), (1 + r + r^2 + r^3 + r^4) + r is composite. hence the first term of the geometric progression is >1. For a(5) a = 5 and r = 2. This is true For all odd n.
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EXAMPLE
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a(3) = (3+6+12) + 2 = 23.
a(4) = (1+2+4+8 ) + 2 = 17.
a(6) = (3+6+12+24+48+96) + 2= 191.
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CROSSREFS
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Cf. A088120.
Sequence in context: A124423 A064187 A112686 this_sequence A144103 A137084 A067256
Adjacent sequences: A088118 A088119 A088120 this_sequence A088122 A088123 A088124
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KEYWORD
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easy,nonn
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AUTHOR
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Amarnath Murthy (amarnath_murthy(AT)yahoo.com), Sep 25 2003
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EXTENSIONS
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More terms from David Wasserman (wasserma(AT)spawar.navy.mil), Jul 25 2005
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