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A219313
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Smallest number k such that LegendreP[2*n, k] is prime.
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1
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3, 7, 7, 3, 41, 5, 89, 23, 21, 35, 55, 5, 181, 511, 241, 83, 709, 401, 3653, 901, 137, 497, 1411, 121, 281, 209, 201, 191, 1667, 89, 39, 181, 233, 2783, 85, 911, 1717, 919, 97, 1163, 1319, 971, 361, 2371, 1573, 121, 817, 733, 1657, 1895, 509, 431, 2399, 1483
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OFFSET
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1,1
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COMMENTS
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LegendreP [2*n, x] is the 2*n th Legendre polynomial of the first kind evaluated at x.
The corresponding primes are in A219315.
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REFERENCES
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M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 798.
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LINKS
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EXAMPLE
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a(1) = 3 because LegendreP[2*1, x] = (3x^2 - 1)/2 = P(x) and P(3) = 13 is prime.
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MATHEMATICA
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Table[k = 0; While[!PrimeQ[LegendreP [2*n, k]], k++]; k, {n, 70}]
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PROG
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(PARI) a(n)=my(P=pollegendre(2*n), k, t); while(denominator(t=subst(P, 'x, k++))>1 || !ispseudoprime(t), ); k \\ Charles R Greathouse IV, Mar 18 2017
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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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