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A227622
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Primes p of the form m^2 + 27.
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3
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31, 43, 127, 223, 283, 811, 1051, 1471, 1627, 2143, 2731, 3163, 3391, 4651, 5503, 6427, 8863, 9631, 16411, 16927, 18523, 23131, 23743, 27583, 28927, 29611, 33151, 37663, 42463, 43291, 44971, 45823, 56671, 65563, 70783, 78427, 80683, 84127, 87643, 106303, 110251, 122527, 123931, 131071
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OFFSET
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1,1
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COMMENTS
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Orders for which residues mod p of the form x^i, i congruent to 0, 1, or 3 (mod 6), form a difference set with parameters (v,k,lambda)=(p,(p-1)/2,(p-3)/4), where x is a primitive root such that 3=x^j, with j congruent to 1 (mod 6). This construction is due to Marshall Hall. Such a difference set has the same parameters as the difference set formed by quadratic residues, that is, the Paley difference set, but is not equivalent to it. Both difference sets give rise to Hadamard matrices of size p+1.
2 is a cube mod p (a particular case of a more general result of Gauss). See A014752.
Primes of the form a^2 + 6*a + 36, where a is an integer.
Let p == 1 (mod 6) be a prime. There are integers c and d, unique up to sign, such that 4*p = c^2 + 27*d^2 [see, for example, Ireland and Rosen, Proposition 8.3.2]. This sequence lists those primes with d = 2. Cf. A005471 (case d = 1) and A349461 (case d = 3).
Primes p of the form m^2 + 27 are related to cyclic cubic fields in several ways:
(1) The cubic polynomial X^3 - p*X + 2*p, with discriminant 4*m^2*p^2, a square, is irreducible over Q by Eisenstein's criteria. It follows that the Galois group of the polynomial over Q is the cyclic group C_3 (apply Conrad, Corollary 2.5).
Note that the roots of the cubic X^3 - p*X + 2*p, are the differences n_0 - n_1, n_1 - n_2 and n_2 - n_0 of the cubic Gaussian periods n_i for the modulus p.
(2) The cubic 2*X^3 + p*(X + 2)^2, with discriminant 64*m^2*p^2, a square, is irreducible over Q by Eisenstein's criteria, and so the Galois group of the polynomial over Q is the cyclic group C_3.
(3) The cubic X^3 - (m-3)*X^2 - 2*(m+3)*X - 8, has discriminant (2*p)^2, a square. (This is the polynomial g_3(m-3, 0, -2; X) in the notation of Hashimoto and Hoshi.) The cubic is irreducible over Q for nonzero m by the Rational Root Theorem and hence the Galois group of the polynomial over Q is the cyclic group C_3. (End)
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REFERENCES
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K. Ireland and M. Rosen, A classical introduction to modern number theory, vol. 84, Graduate Texts in Mathematics. Springer-Verlag. [Prop. 8.3.2, p. 96]
Thomas Storer, Cyclotomy and difference sets. Markham, Chicago, 1967, pages 73-76.
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LINKS
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D. H. Lehmer and Emma Lehmer, The Lehmer Project, Math. of Comp., Vol. 61, No. 203, 1993, pp. 313-317.
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EXAMPLE
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For p=31, using x=3 as primitive root, the set of residues {1,2,3,4,6,8,12,15,16,17,23,24,27,29,30} is a difference set.
2 a cube mod p: 4^3 == 2 (mod 31); 20^3 == 2 (mod 43); 8^3 == 2 (mod 127); 68^3 == 2 (mod 223). - Peter Bala, Nov 19 2021
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MATHEMATICA
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Select[Table[m^2+27, {m, 0, 100}], PrimeQ]
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PROG
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(PARI) for(n=0, 10^3, my(p=n^2+27); if(isprime(p), print1(p, ", "))); \\ Joerg Arndt, Jul 18 2013
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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