Not recognizing infinite sum as zeta function?

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Hi,

Is there a reason why Sage can recognize an infinite series as the zeta function for integers but not fractions (rationals)? For example:

sum(1/t^(2), t, 1, oo), sum(1/t^(3), t, 1, oo), sum(1/t^(11), t, 1, oo)

returns (1/6*pi^2, zeta(3), zeta(11)) But, this:

sum(1/t^(3/2), t, 1, oo), sum(1/t^(5.2), t, 1, oo)

returns just (sum(t^(-3/2), t, 1, +Infinity), sum(t^(-5.2), t, 1, +Infinity))