In the last 15 years, many class number formulas and main conjectures have been proven. Here, we discuss such formulas on the Selmer groups of the three- dimensional adjoint representation ad(φ) of a two- dimensional modular Galois representation φ. We start with the p-adic Galois representation φ_0 of a modular elliptic curve E and present a formula expressing in terms of L(1, ad(φ_0)) the intersection number of the elliptic curve E and the complementary abelian variety inside the Jacobian of the modular curve. Then we explain how one can deduce a formula for the order of the Selmer group Sel(ad(φ_0)) from the proof of Wiles of the Shimura-Taniyama conjecture.
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