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 A091729 Norms of prime elements of Z[sqrt(-5)]. 4
 5, 29, 41, 61, 89, 101, 109, 121, 149, 169, 181, 229, 241, 269, 281, 289, 349, 361, 389, 401, 409, 421, 449, 461, 509, 521, 541, 569, 601, 641, 661, 701, 709, 761, 769, 809, 821, 829, 881, 929, 941, 961, 1009 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Consists of those primes congruent to 1, 5, 9 (mod 20) together with the squares of those primes congruent to -1, -3, -7, -9 (mod 20). Suppose n appears in this sequence. Then the number of prime elements of norm n is 2 if n is 5 or a square and 4 otherwise. REFERENCES David A. Cox, Primes of the form x^2+ny^2, Wiley, 1989. A. Frohlich and M. J. Taylor, Algebraic number theory, Cambridge university press, 1991. LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 PROG (PARI) list(lim)=my(v=List([5]), t); forprime(p=29, lim, t=p%20; if(t==1||t==9, listput(v, p))); forprime(p=11, sqrtint(lim\1), t=p%20; if(t==11||t==13||t==17||t==19, listput(v, p^2))); Set(v) \\ Charles R Greathouse IV, Feb 09 2017 CROSSREFS Cf. A033205 (a subset), A033429, A038872. The sequence of norms of prime ideals in the ring Z[sqrt(-5)] is A091727. Sequence in context: A279393 A182288 A087879 * A033205 A167742 A107151 Adjacent sequences: A091726 A091727 A091728 * A091730 A091731 A091732 KEYWORD easy,nonn AUTHOR Paul Boddington, Feb 02 2004 EXTENSIONS a(43) corrected by Charles R Greathouse IV, Feb 09 2017 STATUS approved

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