A259596 Denominators of the other-side convergents to sqrt(7).

1, 2, 3, 5, 17, 31, 48, 79, 271, 494, 765, 1259, 4319, 7873, 12192, 20065, 68833, 125474, 194307, 319781, 1097009, 1999711, 3096720, 5096431, 17483311, 31869902, 49353213, 81223115, 278635967, 507918721, 786554688, 1294473409, 4440692161, 8094829634

Offset: 0

Author

Clark Kimberling, Jul 20 2015

Keywords

• nonn
• easy
• frac

Comments

Suppose that a positive irrational number r has continued fraction [a(0), a(1), ... ]. Define sequences p(i), q(i), P(i), Q(i) from the numerators and denominators of finite continued fractions as follows:

p(i)/q(i) = [a(0), a(1), ... a(i)] and P(i)/Q(i) = [a(0), a(1), ..., a(i) + 1]. The fractions p(i)/q(i) are the convergents to r, and the fractions P(i)/Q(i) are introduced here as the "other-side convergents" to

r, because p(2k)/q(2k) < r < P(2k)/Q(2k) and P(2k+1)/Q(2k+1) < r < p(2k+1)/q(2k+1), for k >= 0. The closeness of P(i)/Q(i) to r is indicated by |r - P(i)/Q(i)| < |p(i)/q(i) - P(i)/Q(i)| = 1/(q(i)Q(i)), for i >= 0.

Examples

			For r = sqrt(7), 3, 5/2, 8/3, 13/5, 45/17, 82/31, 127/48. A comparison of convergents with other-side convergents:
i    p(i)/q(i)         P(i)/Q(i)    p(i)*Q(i)-P(i)*q(i)
0    2/1   < sqrt(7) <    3/1               -1
1    3/1   > sqrt(7) >    5/2                1
2    5/2   < sqrt(7) <    8/3               -1
3    8/3   > sqrt(7) >   13/5                1
4    37/14 < sqrt(7) <   45/17              -1
5    45/17 > sqrt(7) >   83/31               1
		

Links

• Colin Barker, Table of n, a(n) for n = 0..1000
• Index entries for linear recurrences with constant coefficients, signature (0,0,0,16,0,0,0,-1).

Crossrefs

Cf. A041008, A041009, A259597 (numerators).

Programs

Mathematica

r = Sqrt[7]; a[i_] := Take[ContinuedFraction[r, 35], i];
b[i_] := ReplacePart[a[i], i -> Last[a[i]] + 1];
t = Table[FromContinuedFraction[b[i]], {i, 1, 35}]
u = Denominator[t]
LinearRecurrence[{0,0,0,16,0,0,0,-1},{1,2,3,5,17,31,48,79},40] (* Harvey P. Dale, Jun 03 2017 *)

PARI

Vec(-(x+1)*(x^2-x-1)*(x^4+3*x^2+1)/(x^8-16*x^4+1) + O(x^50)) \\ Colin Barker, Jul 21 2015

Formula

p(i)*Q(i) - P(i)*q(i) = (-1)^(i+1), for i >= 0, where a(i) = Q(i).

a(n) = 16*a(n-4) - a(n-8) for n>7. - Colin Barker, Jul 21 2015

G.f.: -(x+1)*(x^2-x-1)*(x^4+3*x^2+1) / (x^8-16*x^4+1). - Colin Barker, Jul 21 2015