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A239842
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Numbers n such that the Eisenstein integer ((1-ω)^n+1)/(2-ω) has prime norm, where ω = - 1/2 + sqrt(-3)/2.
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0
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5, 11, 31, 37, 47, 53, 97, 163, 167, 509, 877, 1061, 2027, 2293, 3011, 6803, 8423, 13627, 20047, 28411, 50221, 50993, 71453, 152809, 272141, 505823, 1353449
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OFFSET
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1,1
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COMMENTS
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These numbers are sometimes called Eisenstein-Mersenne cofactors EQ(n).
The p-th Eisenstein-Mersenne cofactor can be written as EQ(p) = (3^p + Legendre(3, p) * 3^((p + 1)/2) + 1)/7.
Following an idea of Harsh Aggarwal, some of these numbers have been discovered as by-products of the search for prime Eisenstein-Mersenne norms. The reason of that is the Aurifeuillan factorization of T(k) = 3^(3k) + 1 with k odd. These numbers can be written as T(k) = (3^k + 1)*EM(k)*EQ(k)*7, EM(k) is the norm of the Eisenstein-Mersenne (1-ω)^k-1, while EQ(k) is the norm of ((1-ω)^a[n]+1)/(2-ω).
These numbers have been proved prime only up to exponent a(19) = 20047.
Next term a(28) > 1500000.
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LINKS
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EXAMPLE
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For n = 3: ((1-ω)^31+1)/(2-ω) is an Eisenstein prime because its norm, (3^31-3^16+1)/7 = 88239050462461, is prime.
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PROG
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(PARI) forprime(n=3, 2300, if(ispseudoprime((3^n+kronecker(3, n)*3^((n+1)/2)+1)/7), print1(n ", "))); /* Serge Batalov, Mar 29 2014 */
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CROSSREFS
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Cf. A125743 = Primes p such that (3^p - 3^((p+1)/2) + 1)/7 is prime.
Cf. A125744 = Primes p such that (3^p + 3^((p+1)/2) + 1)/7 is prime.
Cf. A066408 = Numbers n such that the Eisenstein integer has prime norm.
Cf. A124112 = Numbers n such that ((1+I)^n+1)/(2+I) is a Gaussian prime.
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KEYWORD
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more,nonn,hard
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AUTHOR
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STATUS
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approved
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