A092205 - OEIS

A092205

Number of units in the imaginary quadratic field Q(sqrt(-n)).

4, 2, 6, 4, 2, 2, 2, 2, 4, 2, 2, 6, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 2, 4, 2, 6, 2, 2, 2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 6, 4, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 6, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 4, 2, 2, 2, 2, 2

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OFFSET

1,1

COMMENTS

Sequence of n such that a(n)=2 gives A092206; a(n)=4 gives A000290; a(n)=6 gives A033428. - Marc LeBrun, Apr 12 2006

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..10000

Eric Weisstein's World of Mathematics, Unit

FORMULA

a(A005117(n)) = A236213(n). - Jonathan Sondow, Jan 29 2014

EXAMPLE

For n=1, the units are +/-1, +/-i, so a(1) = 4.

For n=3, the units are +/-1, +/-w, +/-w^2, where w is a cube root of unity, so a(3) = 6. [Corrected by Jonathan Sondow, Jan 29 2014]

MAPLE

A092205 := proc(n) if(type(sqrt(n), integer))then return 4: elif(n mod 3 = 0 and type(sqrt(n/3), integer))then return 6: else return 2: fi: end: seq(A092205(n), n=1..105); # Nathaniel Johnston, Jun 26 2011

MATHEMATICA

a[n_] := Which[ IntegerQ[ Sqrt[n] ], 4, Mod[n, 3] == 0 && IntegerQ[ Sqrt[n/3] ], 6, True, 2]; Table[a[n], {n, 1, 105}] (* Jean-François Alcover, Oct 30 2012, after Nathaniel Johnston *)

PROG

(PARI) a(n)=if(issquare(n), return(4)); if(n%3==0&&issquare(n/3), 6, 2) \\ Charles R Greathouse IV, Oct 30 2012

CROSSREFS

Cf. A092206, A000290, A033428, A236213.

Sequence in context: A273667 A187109 A242599 * A272101 A059853 A136527

Adjacent sequences: A092202 A092203 A092204 * A092206 A092207 A092208

KEYWORD

nonn,easy

AUTHOR

Eric W. Weisstein, Feb 24 2004

STATUS

approved