In this post, I will talk about the connection between continued fraction expansion of sqrt(d) (where d is a non-square positive integer) and the group of units in the ring of integers of a quadratic number field, namely Q ( sqrt(d) ) , which consists of elements of the form a+b*sqrt(d) , where a and b are rationals. There is a few special elements in the field Q ( sqrt(d) ) , the first one is the integral element , which is a root of a monic polynomial with integer coefficients. Note that a monic polynomial is a polynomial whose highest term has a coefficient of 1 . For example, consider x = 2 - sqrt(5) in Q ( sqrt(5) ) , and we can find the monic quadratic polynomial that x is a root of: x - 2 = sqrt(5) (x − 2) 2 = 5 x 2 − 4x − 1 = 0 Since x 2 − 4 x − 1 is a monic polynomial, and x is one of its roots, we conclude that x is an integral element of Q ( sqrt(5) ) . Secondly, the ring of integers is the ring of all integral elements contained in Q ( sqrt(d) ) , an
A finite continued fraction (FCF) is an expression of the form: where a 0 , a 1 , ..., a n are real numbers, all of which except possibly a 0 are positive. Such a fraction is called simple if all the a i are integers. The expression is denoted as [ a 0 ; a 1 , . . . , a n ]. Any rational number can be expressed as a FCF. Recall that the reciprocal of a number is simply obtained by dividing 1 by that number. Consider the fraction 45/16, using reciprocals, this fraction can be written as: Hence we obtain 45/16 = [2; 1, 4, 3] . Euclid's Algorithm is well known for finding the highest common factor between two integers, where we repeatedly employ division by remainder. The procedure is as follows: given two natural numbers a, b with a > b > 0 , we can write a = q 1 b + r 1 where b >r 1 ≥ 0 b = q 2 r 1 + r 2 where r 1 > r 2 ≥ 0 r 1 = q 3 r 2 + r 3 where r 2 > r 1 ≥ 0 in general r t = q t +2 r t +1 + r t +2 with r t +1 >
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