(Solved): 1. Beltrami identity Consider the integral functional J[y]=x1x2dxF(x;y(x),y(x)). Sho ...
1. Beltrami identity Consider the integral functional J[y]=?x1?x2??dxF(x;y(x),y?(x)). Show that when the integrand does not depend explicitly on x, then F?y?(x)?y??F?=C where C is an undetermined constant. Hint: Do not start from scratch. Use the Euler-Lagrange equation, and use that dxdF?=?x?F?+?x?y??y?F?+?x?y???y??F?