Asymptotic Behaviour of the Error of Polynomial Approximation of Functions Like $\vert x\vert^{\alpha+i\beta}$

New asymptotic relations between the $L_p$-errors of best approximation of univariate functions by algebraic polynomials and entire functions of exponential type are obtained for $p\in (0,\iy]$. General asymptotic relations are applied to functions $\vert x\vert^{\al+i\be},\,\vert x\vert^{\al}\cos(\be\log\vert x\vert)$, and $\vert x\vert^{\al}\sin(\be\log\vert x\vert)$.