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A090464
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Smallest number m such that n followed by m sevens yields a prime or -1 if no solution exists or has been found for n.
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2
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1, 0, 0, 1, 0, 1, 0, 2, 1, 1, 0, 1, 0, -1, 1, 1, 0, 2, 0, 6, -1, 1, 0, 2, 1, 2, 1, -1, 0, 1, 0, 5, 1, 1, -1, 1, 0, 2, 1, 12, 0, -1, 0, 3, 1, 1, 0, 1, -1, 2, 8, 7, 0, 1, 1, -1, 1, 1, 0, 1, 0, 2, -1, 1, 2, 5, 0, 3, 2, -1, 0, 1, 0, 2, 1, 3, -1, 1, 0, 3, 4, 1, 0, -1, 1, 2, 1, 1, 0, 1, -1, 2, 1, 1
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,8
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COMMENTS
| a(n) = 0 if n is already prime. a(n) = -1 for n = any multiple of 7 other than 7 itself. Each multiple of 7 has been tested out to 2000 7's with no result found. The first eight values of n which are not multiples of 7 for which no answer has yet been found are 95, 480, 851, 891, 957, 1184, 1261, 1881. 95 has been tested out to 2100 7's, 1881 has been tested out to 1750 7's, the others have been tested out to 2000 7's. Pending solutions for these values of n, the first 10 record holders are currently 1, 8, 20, 40, 120, 128, 225, 260, 296, 711 with the values 1, 2, 6, 12, 16, 18, 56, 182, 434, 1648 respectively.
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EXAMPLE
| a(20)=6 because six 7's must be appended to 20 before a prime is formed (20777777). a(14) = -1 because no matter how many 7's are appended to 14, the resulting number is always divisible by 7 and can therefore not be prime.
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CROSSREFS
| Cf. A083747 (The Wilde Primes, i.e. same operation using ones), A090465 (using nines), A090584 (using threes).
Sequence in context: A133300 A178779 A144451 * A196049 A194821 A044934
Adjacent sequences: A090461 A090462 A090463 * A090465 A090466 A090467
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KEYWORD
| base,sign
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AUTHOR
| Chuck Seggelin (barkeep(AT)plastereddragon.com), Dec 02 2003
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