  The paper is devoted to the construction of an optimal interpolation formula
in $K_2(P_2)$ Hilbert space. Here the interpolation formula consists of a
linear combination $\sum_{\beta=0}^NC_{\beta}(z)\varphi(x_\beta)$ of given
values of a function $\varphi$ from the space $K_2(P_2)$. The difference
between functions and the interpolation formula is considered as a linear
functional called the error functional. The error of the interpolation formula
is estimated by the norm of the error functional. We obtain the optimal
interpolation formula by minimizing the norm of the error functional by
coefficients $C_{\beta}(z)$ of the interpolation formula. The obtained optimal
interpolation formula is exact for trigonometric functions $\sin\omega x$ and
$\cos\omega x$. At the end of the paper, we give some numerical results which
confirm our theoretical results.
