# Page:EB1911 - Volume 08.djvu/126

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DETAINER—DETERMINANT

DETAINER (from detain, Lat. detinere), in law, the act of keeping a person against his will, or the wrongful keeping of a person's goods, or other real or personal property. A writ of detainer was a form for the beginning of a personal action against a person already lodged within the walls of a prison; it was superseded by the Judgment Act 1838.

DETERMINANT, in mathematics, a function which presents itself in the solution of a system of simple equations.

1. Considering the equations

 ax + by + cz = d, a′x + b′y + c′z = d′, a″x + b″y + c″z = d″,

and proceeding to solve them by the so-called method of cross multiplication, we multiply the equations by factors selected in such a manner that upon adding the results the whole coefficient of y becomes = 0, and the whole coefficient of z becomes = 0; the factors in question are b′c″ - b″c′, b″c - bc″, bc′ - b′c (values which, as at once seen, have the desired property); we thus obtain an equation which contains on the left-hand side only a multiple of x, and on the right-hand side a constant term; the coefficient of x has the value

a(b′c″ - b″c′) + a′(b″c - bc″) + a″(bc′ - b′c),

and this function, represented in the form

 a, b, c , a′, b′, c′ a″, b″, c″

is said to be a determinant; or, the number of elements being 3², it is called a determinant of the third order. It is to be noticed that the resulting equation is

 a, b, c x = d, b, c a′, b′, c′ d′, b′, c′ a″, b″, c″ d″, b″, c″

where the expression on the right-hand side is the like function with d, d′, d″ in place of a, a′, a″ respectively, and is of course also a determinant. Moreover, the functions b'c″ - b″c′, b″c - bc″, bc′ - b′c used in the process are themselves the determinants of the second order

 b′, c′ , b″, c″ , b, c . b″, c″ b, c b′, c′

We have herein the suggestion of the rule for the derivation of the determinants of the orders 1, 2, 3, 4, &c., each from the preceding one, viz. we have

 a = a,

 a, b = a b′ - a′ b . a′, b′

 a, b, c = a b′, c′ + a′ b″, c″ + a″ b, c , a′, b′, c′ b″, c″ b, c b′, c′ a″, b″, c″

 a, b, c, d = a b′, c′, d′ - a′ b″, c″, d″ + a″ b″′, c″′, d″′ - a′″ b, c, d , a′, b′, c′, d′ b″, c″, d″ b′″, c′″, d′″ b, c, d b′, c′, d′ a″, b″, c″, d″ b′″, c′″, d′″ b, c, d; b′, c′, d′ b″, c″, d″ a′″, b′″, c′″, d′″

and so on, the terms being all + for a determinant of an odd order, but alternately + and - for a determinant of an even order.

2. It is easy, by induction, to arrive at the general results:—

A determinant of the order n is the sum of the 1.2.3...n products which can be formed with n elements out of n² elements arranged in the form of a square, no two of the n elements being in the same line or in the same column, and each such product having the coefficient ± unity.

The products in question may be obtained by permuting in every possible manner the columns (or the lines) of the determinant, and then taking for the factors the n elements in the dexter diagonal. And we thence derive the rule for the signs, viz. considering the primitive arrangement of the columns as positive, then an arrangement obtained therefrom by a single interchange (inversion, or derangement) of two columns is regarded as negative; and so in general an arrangement is positive or negative according as it is derived from the primitive arrangement by an even or an odd number of interchanges. [This implies the theorem that a given arrangement can be derived from the primitive arrangement only by an odd number, or else only by an even number of interchanges,—a theorem the verification of which may be easily obtained from the theorem (in fact a particular case of the general one), an arrangement can be derived from itself only by an even number of interchanges.] And this being so, each product has the sign belonging to the corresponding arrangement of the columns; in particular, a determinant contains with the sign + the product of the elements in its dexter diagonal. It is to be observed that the rule gives as many positive as negative arrangements, the number of each being = ½ 1.2...n.

The rule of signs may be expressed in a different form. Giving to the columns in the primitive arrangement the numbers 1, 2, 3 ... n, to obtain the sign belonging to any other arrangement we take, as often as a lower number succeeds a higher one, the sign -, and, compounding together all these minus signs, obtain the proper sign, + or - as the case may be.

Thus, for three columns, it appears by either rule that 123, 231, 312 are positive; 213, 321, 132 are negative; and the developed expression of the foregoing determinant of the third order is

= ab′c″ - ab″c′ + a′b″c - a′bc″ + a″bc′ - a″b′c.

3. It further appears that a determinant is a linear function[1] of the elements of each column thereof, and also a linear function of the elements of each line thereof; moreover, that the determinant retains the same value, only its sign being altered, when any two columns are interchanged, or when any two lines are interchanged; more generally, when the columns are permuted in any manner, or when the lines are permuted in any manner, the determinant retains its original value, with the sign + or - according as the new arrangement (considered as derived from the primitive arrangement) is positive or negative according to the foregoing rule of signs. It at once follows that, if two columns are identical, or if two lines are identical, the value of the determinant is = 0. It may be added, that if the lines are converted into columns, and the columns into lines, in such a way as to leave the dexter diagonal unaltered, the value of the determinant is unaltered; the determinant is in this case said to be transposed.

4. By what precedes it appears that there exists a function of the n² elements, linear as regards the terms of each column (or say, for shortness, linear as to each column), and such that only the sign is altered when any two columns are interchanged; these properties completely determine the function, except as to a common factor which may multiply all the terms. If, to get rid of this arbitrary common factor, we assume that the product of the elements in the dexter diagonal has the coefficient +1, we have a complete definition of the determinant, and it is interesting to show how from these properties, assumed for the definition of the determinant, it at once appears that the determinant is a function serving for the solution of a system of linear equations. Observe that the properties show at once that if any column is = 0 (that is, if the elements in the column are each = 0), then the determinant is = 0; and further, that if any two columns are identical, then the determinant is = 0.

5. Reverting to the system of linear equations written down at the beginning of this article, consider the determinant

 ax + by + cz - d, b, c ; a′x + b′y + c′z - d′, b′, c′ a″x + b″y + c″z - d″, b″, c″

it appears that this is

 = x a, b, c + y b, b, c + z c, b, c - d, b, c ; a′, b′, c′ b′, b′, c′ c′, b′, c′ d′, b′, c′ a″, b″, c″ b″, b″, c″ c″, b″, c″ d″, b″, c″

viz. the second and third terms each vanishing, it is

 = x a, b, c - d, b, c . a′, b′, c′ d′, b′, c′ a″, b″, c″ d″, b″, c″

But if the linear equations hold good, then the first column of the

1. The expression, a linear function, is here used in its narrowest sense, a linear function without constant term; what is meant is that the determinant is in regard to the elements a, a′, a″, ... of any column or line thereof, a function of the form Aa + A′a′ + A″a″ + ... without any term independent of a, a′, a″ ...