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A225953 Table for period length of periods of primitive reduced binary quadratic forms with discriminants D(n) = A079896(n). 11
2, 2, 2, 2, 2, 6, 2, 2, 2, 2, 2, 4, 4, 2, 2, 2, 4, 4, 6, 6, 2, 10, 2, 2, 2, 2, 2, 2, 10, 2, 4, 4, 6, 6, 2, 2, 2, 2, 6, 6, 6, 2, 4, 4, 2, 2, 18, 6, 6, 2, 2, 2, 2, 6, 6, 6, 2, 6, 6, 14, 4, 4, 2, 2, 4, 4, 2, 2, 18, 6, 6, 2, 6, 6, 4, 4, 2, 2, 14, 4, 4, 14, 10, 2, 2, 4, 4, 2, 2, 8, 8, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

The indefinite binary quadratic forms [a,b,c] have discriminant D := b^2 - 4*a*c > 0, not a square, given in A079896.

Primitive forms satisfy gcd(a,b,c) = 1. For the definition of reduced binary quadratic forms see a comment under A087048.

The number of periods of equivalent primitive reduced forms is given in A087048 (the class number).

Here the lengths of these periods is recorded. The computation is based on the book by Scholz and Schoeneberg. The row sums give A082174(n), the number of primitive reduced forms for D(n).

Two forms [a,b,c] and [a',b',c'] are properly equivalent if the 2 x 2 coefficient matrices A := [[a,b/2],[b/2,a]] and A' := [[a',b'/2],[b'/2,a']] satisfy A' = S^{-1,T} A S^{-1} with some matrix S, det S = +1 (T stands for transposed). The indeterminates (x,y) and (x',y') which represent the same number k = (x,y) A (x,y)^T = (x',y') A' (x',y')^T are related then by (x',y')^T = S (x,y)^T.

For the periods of primitive reduced forms for D(n), n = 0, ..., 100, see the link. See also the Buell reference, with the examples on p. 30, giving the periods for n = 0, ..., 19. They coincide with the ones given in the link up to the cyclic order in the periods.

All period lengths are even. See Buell, Proposition 3.6 on p. 24.

REFERENCES

D. A. Buell, Binary quadratic forms,1989, Springer, especially Ch. 3.

A. Scholz and B. Schoeneberg, Einfuehrung in die Zahlentheorie, 5. Aufl., de Gruyter, Berlin, New York, 1973, ch. 31, pp. 112 ff.

LINKS

Table of n, a(n) for n=0..91.

Wolfdieter Lang, Table for n = 0..100 and more.

Wolfdieter Lang, Periods of Indefinite Binary Quadratic Forms, Continued Fractions and the Pell +/-4 Equations.

FORMULA

a(n,k), n >= 0, k = 1, 2, ..., A087048(n), is the length of the period of the k-th primitive reduced forms for discriminant D(n) = A079896. The order in row n is nonincreasing.

EXAMPLE

The irregular table a(n,k) begins:

n/k  1   2  ...   D(n)    A087048(n)   A082174(n)

0:   2              5        1             2

1:   2              8        1             2

2:   2   2         12        2             4

3:   2             13        1             2

4:   6             17        1             6

5:   2             20        1             2

6:   2   2         21        2             4

7:   2   2         24        2             4

8:   4   4         28        2             8

9:   2             29        1             2

10:  2   2         32        2             4

11:  4   4         33        2             8

12:  6             37        1             6

13:  6   2         40        2             8

14: 10             41        1            10

15:  2   2         44        2             4

16:  2   2         45        2             4

17:  2   2         48        2             4

18: 10             52        1            10

19:  2             53        1             2

20:  4   4         56        2             8

... for rows up to n = 100 see the link.

a(0) = 2 because there are 2 = A082174(n) primitive reduced forms with discriminant D = 5, namely  [-1, 1, 1] and  [1, 1, -1], which are equivalent, and they give 1 = A087048(0) period [[[-1, 1, 1],[1, 1, -1]]] of length 2 = a(0,1).

Row n=2 is  2  2, because there are 4 primitive reduced forms for D = 12, appearing in 2 periods, each of length 2: [[[-1, 2, 2], [2, 2, -1]], [[1, 2, -2], [-2, 2, 1]]].

CROSSREFS

Cf. A079896, A082174, A087048.

Sequence in context: A266995 A051887 A139516 * A278247 A160762 A112968

Adjacent sequences:  A225950 A225951 A225952 * A225954 A225955 A225956

KEYWORD

nonn,tabf

AUTHOR

Wolfdieter Lang, May 27 2013

STATUS

approved

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Last modified June 2 11:20 EDT 2020. Contains 334771 sequences. (Running on oeis4.)