P. Deift et R. Killip, On the absolutely continuous spectrum of one-dimensional Schrodinger operators with square summable potentials, COMM MATH P, 203(2), 1999, pp. 341-347
For continuous and discrete one-dimensional Schrodinger operators with squa
re summable potentials, the absolutely continuous part of the spectrum is e
ssentially supported by [0, infinity) and [-2, 2] respectively. This fact i
s proved by considering a priori estimates for the transmission coefficient
.