This equation can be rewritten as ∂ ∂ 2 ( r u ) ∂ ∂ t 2 − − c 2 ∂ ∂ 2 ( r u ) ∂ ∂ r 2 = 0 , where the quantity ru satisfies the one-dimensional wave equation. Therefore, there are solutions in the form u ( r , t ) = 1 r F ( r − − c t ) + 1 r G ( r + c t ) , where F and G are general solutions to the one-dimensional wave equation and can be interpreted as respectively an outgoing and incoming spherical waves. The outgoing wave can be generated by a point source, and they make possible sharp signals whose form is altered only by a decrease in amplitude as r increases (see an illustration of a spherical wave on the top right). Such waves exist only in cases of space with odd dimensions.