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A260349
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a(n) = min(k : A046067((k+1)/2) = n).
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2
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1, 3, 7, 17, 55, 59, 19, 167, 31, 311, 289, 227, 351, 203, 379, 197, 103, 1253, 829, 335, 211, 353, 649, 437, 1921, 1853, 2869, 917, 361, 263, 283, 1637, 1213, 3353, 1519, 797, 241, 1691, 259, 1391, 2503, 1109, 3859, 1857
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OFFSET
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0,2
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COMMENTS
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a(n) is the first odd number k for which k * 2^i + 1 is prime when i = n but composite for all i: 0 <= i < n, or 0 if no such k exists. Thus it is the first k for which A046067((k+1)/2) = n, and therefore also the first k for which you need to test the primality of exactly n values to show that it is not a Sierpiński number.
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LINKS
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EXAMPLE
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7 * 2^i + 1 is composite for i < 2 (with values 8, 15) but prime for i = 2 (29); the smaller odd numbers 1, 3 and 5 each yield a prime for smaller i, so a(2) = 7.
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PROG
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(PARI) a(n)=forstep(k=1, +oo, 2, for(i=0, n-1, ispseudoprime(k<<i+1)&&next(2)); ispseudoprime(k<<n+1)&&return(k)) \\ Jeppe Stig Nielsen, Jul 06 2020
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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