Cut and Paste Invariants and Duality: zeta(-1), zeta(2) and SL_2Z
Here’s something enticing and strange: the euler characteristic \( \chi(SL_2(\mathbb{Z})) = \zeta(-1), \) and the tamagawa measure of \( \mu(SL_2(\mathbb{Z})\backslash SL_2(\mathbb{R}) = \zeta(2)\). That is, these apriori unrelated invariants of \( SL_2(\mathbb{Z}), \) the goodest boy and lovely example case, are dual zeta values. Is it a general pattern? Let’s define and explain these results to spiritually prepare ourselves for the general picture!