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A260350
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Define g(k) = min(n: n >= 0, 2^n + k prime). Then a(n) = min(odd k: g(k) = n).
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4
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1, 3, 7, 23, 31, 47, 199, 83, 61, 257, 139, 953, 991, 647, 1735, 383, 511, 1337, 1069, 713, 271, 1937, 3223, 5213, 751, 8477, 4339, 353, 1501, 287, 829, 1553, 2371, 1811, 11185, 3023, 7381, 7937, 6439, 1433, 13975, 2897, 4183
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OFFSET
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0,2
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COMMENTS
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Previous name: a(n) = min(k : A067760((k-1)/2)) = n.
a(n) is the first odd number k for which 2^m + k is the first prime value, as m ranges from 0 to n, or 0 if no such k exists. Thus it is the first k for which A067760((k-1)/2) = n, and therefore also the first k for which you need to test primality of exactly n values to show that it is not a dual Sierpiński number.
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LINKS
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FORMULA
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EXAMPLE
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2^i + 7 is composite for i < 2 (with values 8, 9) but prime for i = 2 (11); 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) g(k) = {my(j=0); while (!isprime(2^j+k), j++); j; }
a(n) = {my(k = 1); while(g(k) != n, k+=2); k; } \\ Michel Marcus, Apr 07 2018
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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