Abstract:
Consider finite Euler product $$
\zeta_{m}(s)\,=\,\prod_{k=1}^{m}(1-p_k^{-s})^{-1},
$$
where $p_1,\,\dots,\,p_m$ are the initial primes, and finite xi function $$
g(s)\,=\,\pi^{-\frac{s}{2}}(s-1)\Gamma\bigl({s}/{2}+1\bigr)
$$
is the factor from the functional equation. Modified symmetrized
finite xi function $$
\xi^{ :=}_{{m}}(s) \! =\!s^m(1\!-\!s)^m\big(\xi_{m}(s)\!+\!\xi_{m}(1-s)\big)
$$
trivially satisfies the functional equation
$\xi^{ :=}_{{m}}(s)=\xi^{ :=}_{{m}}(1-s)$.
All poles $q_1,\,q_2,\dots$ of this function are simple;
let $r_1,\,r_2,\dots$ be corresponding residues, so the difference
$$
\xi^{ :\text{reg}=}_{{m}}(s)\,=\,\xi^{ :=}_{{m}}(s)-\sum_{k=1}^\infty r_k/(s-q_k).
$$ Regularized finite Euler product $$
\zeta^{\approx}_{{m}}(s)\,=\,\xi^{ :\text{reg}=}_{{m}}(s)/\big( s^m(1-s)^m g(s)\big)
$$
gives surprisingly good approximations to the values and zeroes of the zeta function.
Example 1. The least (in absolute value) non-real zero of
function
$\zeta^{\approx}_{{1}}(s)$ (which is defined via single
Euler factor $(1-2^{-s})^{-1}$) differs from the least
non-trivial zero of the zeta function less than by $10^{-6}$.
Example 2. The three first Euler factors produce more than 30
correct decimal digits of $\zeta(1/2+100i)$.
For more examples visit
http://logic.pdmi.ras.ru/~yumat/personaljournal/ eulereverywhere.