  The shift of a maxon-roton He-4 excitation spectrum E via the He-3 admixture
is considered theoretically. The Bogoliubov formula for E(\rho, g) as a
function of Bose-condensate density \rho and of He-4/He-4 interaction g is used
as an element of partition function formalism of the whole system, presented in
the form of a path (functional) integral. The dependence of \rho on g,
He-4/He-3 interaction \lambda as well as on the pressure P, induced by He-3
admixture, is taken into account. The new model of g, suitable for analytical
calculations, is used for a description of a change of P within the constant
volume. The cases of positive and negative shifts are found as the result of
competition between interactions g and \lambda.
