hollow lintel
F1'(x,y) is basically dF(x,y)/dx (which assumes y is constant);
F2'(x,y) is basically dF(x,y)/dy (which assumes x is constant).
F11''(x,y) is basically 2nd Differentiation of F1'(x,y)
F22'(x,y) is basically 2nd Differentiation of F2'(x,y)
But what is F12''(x,y) or F21"(x,y) ?
red leaf
