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A363709
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For n >= 0, a(n) is the least integer i >= 0 such that n + (n + 1) + ... + (n + i) is a prime number or a(n) = -1 if no such i exists.
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0
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2, 1, 0, 0, -1, 0, 1, 0, 1, 1, -1, 0, -1, 0, 1, 1, -1, 0, 1, 0, 1, 1, -1, 0, -1, -1, 1, -1, -1, 0, 1, 0, -1, 1, -1, 1, 1, 0, -1, 1, -1, 0, -1, 0, 1, -1, -1, 0, 1, -1, 1, 1, -1, 0, 1, -1, 1, -1, -1, 0, -1, 0, -1, 1, -1, 1, -1, 0, 1, 1, -1, 0, -1, 0, 1, 1, -1, -1, 1, 0, -1
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OFFSET
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0,1
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COMMENTS
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Shortest arithmetic sequence with initial term n and difference 1 that sums to a prime number.
For n = 0, a(n) = 2.
For n >= 1, a(n) < 2.
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LINKS
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FORMULA
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((i + 1)*(i + 2*n))/2 = p, p prime number.
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EXAMPLE
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n = 4: no such i exists, thus a(4) = -1.
n = 5: n is prime, i = 0, thus a(5) = 0.
n = 6: n + (n + 1) = 6 + 7 = 13 is prime, i = 1, thus a(6) = 1.
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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STATUS
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approved
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