We obtain some upper and lower estimates for the sequences of the Lebesgue functions and constants of the Whittaker operatorsnx − kπ f kπ n for continuous functions. We give an analog of Nevai's formula for the Lagrange-Chebyshev and Lagrange-Laguerre interpolation polynomials for the operators under consideration. Its "local" version is established.

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We obtain asymptotic formulas for the values of second-order differential operators with the coefficient of bounded variation depending on the spectral parameter. We give a counterexample showing that the requirement of boundedness for the variation is essential for the preservation of the error of the so-obtained asymptotic formulas.

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