A simple treatment of the sine-Gordon equation

To avoid difficulties due to the complexity of the Lax pair of the sine-Gor don equation, the orthogonality relations of the squared Jost solutions is derived simply using the 1+1 dimensional Green's theorem. The direct pertur bation theory for sine-Gordon in the laboratory reference is re-developed, correcting some numerical coefficients in published orthogonality relations hips. (C) 2001 American Institute of Physics.