Functionals of Curves Admitting One-Parameter Groups of Infinitesimal Point Transformation

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which are of the form 4n + 1 and for R = 7 those cells are punched out which correspond to primes of the forms 28n + 1, 3, 9, 19,25,27. When these stencils are superposed the holes which shine through both stencils correspond to primes which belong to both sets of forms. The first stencil will shut out approximately one half the list of primes, and the second will shut out approximately half of those that remain; so that the discovery of as many as 12 residues will serve to exclude all but about 12 trial divisors for a number as great as the square of 48,593 or 2,361,279,649. For numbers of this order a sufficient number of quadratic residues less in absolute value than 1000 will not be difficult to obtain by well-known methods, the most effective of which is by expanding the square root of the number in a continued fraction. A machine for cutting the stencils from the master-stencil is easily devised. The labor of proof-reading or of type-setting with all the errors involved in those operations is completely eliminated. The device has one very important advantage over a factor table in that while a factor table is of no service at all for a number just beyond its range the system of stencils will be very useful for numbers far beyond the limit indicated. Moreover, any additional information regarding the character of the factors is not difficult to make use of, which advantage is not enjoyed by other powerful methods such as the representation of the number by binary quadratic forms.

FUNCTIONALS OF CURVES ADMITTING ONE-PARAMETER GROUPS OF INFINITESIMAL POINT TRANSFORMATIONS' By ARIsToTLo D. MIcHAi, DUPARTM1ENT Olt MATHEMATICS, THE Rics INs5ITiuT Communicated November 5, 1924

The purpose of this abstract is to study functionals of closed plane curves admitting one-parameter continuous groups of infinitesimal point transformations. Necessary and sufficient conditions are given. Extensions and generalizations of this invariant theory to functionals of closed plane curves depending in particular on a point on the curve and to functionals of n - 1 spreads in n dimensions are indicated. The theory of functional invariants may be divided into two parts: one, which seeks functionals admitting one-parameter groups of functional infinitesimal transformations ;2 and another, the object of this paper, which seeks functionals admitting one-parameter groups of ordinary infinitesimal transformations.3

VoL. 11, 1925

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We consider a group of continuous transformations defined by

dx

dy

Z(M)

71(M)= t(1

M denoting the variable point of the curve and t the parameter of the group of transformations; and Z(M) and rI(M) denoting real analytic functions of x and y. Let 4X,'[C/M] stand for the normal functional derivative of a functional 11 [C] of closed plane curves C taken at a point M on C; and let 5n(M) stand for the variation of the normal to the curve C (the inward direction considered positive). Assume the functionals 4 [C] to have a variation in the normal form

a1 [C =f

[C/M] An(M) ds,

(2)

c

A necessary and sufficient condition that such a functional 4 [C] admit the group of transformations (1) is that

fb[C/MI 5n(M) ds =- 0

(3)

c

for all closed curves C, where An(M) is obviously given by

An(M) = [t (M) cos (x, n) + rn (M) cos (y, n)] At. To get an explicit solution of the functional equation (3) we assume J [C] to be developable in an expansion

cI4C]=o(o) + E ...r f(i)(M1, M2... ,Mj)de d623...d6j, ij

! 6

6

(4)

(i) 6

in which

((Ml, ...,Mi) I< Ks (K > O) (i

1, 2,3. ,(t) is symmetric4 in its arguments M1; M2, . .., M,; d6h designates the ele=

ment of area which corresponds to the point Mh, and 6 is the area within the closed curve C. By functional analysis we can demonstrate the theorem: THZORZM I. If b[C] is developable in an expansion (4), then a necessary

and sufficient condition that TX [C] admit the group of transformations defined by (1) is that the p s in P [C] satisfy the linear partial differential equation of the first order.

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M, (M3)A} N {t(M) 8"' Ml,.+ s$+ +... + t(1 1(5) M))Ff' n(P (M .7~~~~~~~~~~b(i = 1, 2, 3. )

Consequently we may assert that there always exist functionals of closed plane curves of form (4) admitting a given arbitrary continuous one-parameter group of transformations defined by (1). The application of this general theory to the case of some of the more important groups (such as the projective and conformal) furnishes ample exercises in solutions of partial differentia lequations. We, however, note in passing that for area preserving groups the problem reduces to the solution of homogeneous partial differential equations. The reader will have no difficulty in seeing that by means of one of Poincare's classical theorems5 conditions (5) can be interpreted in terms of absolute integral invariants in hyperspaces. We have the following theorem which gives information as to the most general invariant 4[C] of type (4). THioRuM II. If

1[C]

=

(o2 + i=1

f

f4(PO

6

6

(M1,...,Mi) d6... . d6i

(6)

(i)

is a functional of closed plane curves admitting the group of transformations

defined by (1) and if It (M1,M2,. .. ,M) is the most general symmetric function of M1, M2,. . . ,Mi which admits the group of transformations defined by

dx1

dxi {(M,)

dx2

{(MI) -(M2)

dyi

t1(MI)

dy2

n(M2) -

dyi

(Mi) (i = 1,2, 3,....

then the most general functional of closed plane curves of the type (4) admitting the group of transformations defined by (1) can be written as