I've recently been learning about the special values of symmetric square $L$-functions of modular forms.


If $f$ is a cuspidal modular eigenform (of some weight $k \ge 2$) then its symmetric square $L$-function is the $L$-function given by $\prod\_{\text{$\ell$ prime}} D\_\ell(\ell^{-s})^{-1}$, where for almost all $\ell$ we have $D\_\ell(X) = (1 - \alpha^2 X)(1 - \alpha \beta X)(1 - \beta^2 X)$, $\alpha, \beta$ being as usual the roots of the Hecke polynomial $X^2 - a\_\ell(f) X + \ell^{k-1} \varepsilon\_f(\ell)$.


Now, it seems to me that the critical values of $L(\operatorname{Sym}^2 f, s)$ (in the sense of Deligne) should be the odd integers in the range $1 \le s \le k-1$, and the even integers in the range $k \le s \le 2k-2$. If you twist the symmetric square by a Dirichlet character $\chi$ with $\chi(-1) = -1$, then the parity flips and you see even integers in $\{1, \dots, k-1\}$ and odds in $\{k, \dots, 2k-2\}$.


What's puzzling me is an assertion in Hida's paper "P-adic L-functions for base-change lifts from GL(2) to GL(3)" (see [link](http://www.jmilne.org/math/Books/AA1988b.pdf), page 93 onwards). Hida works with a different $L$-function, a shifted version of the adjoint $L$-function; so his $L$-function is $L(\operatorname{Sym}^2(f) \otimes \varepsilon\_f^{-1}, s)$ in my notation. Thus the critical values of Hida's $L$-function, by my computation, should be the integers in $\{1, \dots, k-1\}$ with the opposite parity to $k$, and the integers in $\{k, \dots, 2k-2\}$ with the same parity as $k$ (so the "near-central" values $k-1$ and $k$ are always critical).


However, Hida states on p96 of his paper that the critical values of his $L$-function are the odd integers in $\{1, \dots, k-1\}$ and the even ones in $\{k, \dots, 2k-2\}$, regardless of the parity of $k$. Have I got this computation wrong, or is this a mistake in Hida's paper?



Which values of symmetric square $L$-functions are critical?
I have not yet checked Hida's normalization, but I think you're right about the critical range.


The critical range of the symmetric square L-function is studied in C.G. Schmidt's paper "p-adic measures attached to automorphic representations of GL(3)".


Using the notation of this paper (see bottom of page 603), for any Dirichlet character $\chi$, we have $L(\mathrm{Sym}^2 f \otimes \chi, s) = L(s-k+1, \Sigma \otimes (\chi \varepsilon\_f)^{-1})$. (I'm simplifying notation a bit, since the $\chi$ in the RHS should be a Grössencharakter.)


Now the critical range is given in Lemma 2.1 of this paper (up to a shift). This gives the integers in $\{1,2,\ldots,k-1\}$ with opposite parity to $k$, together with the integers in $\{k,k+1,\ldots,2k-2\}$ with same parity as $k$.


This is consistent with what we assert about $s=k-1$ in the new version of [my preprint with Chida](http://perso.ens-lyon.fr/francois.brunault/recherche/Rankin-Selberg.pdf) (see Remark 5.4; the weight is $k+2$ in our article).


