Published online by Cambridge University Press: 21 November 2019
Fix an odd prime p. Let $\mathcal{D}_n$ denote a non-abelian extension of a number field K such that
$K\cap\mathbb{Q}(\mu_{p^{\infty}})=\mathbb{Q}, $ and whose Galois group has the form
$ \text{Gal}\big(\mathcal{D}_n/K\big)\cong \big(\mathbb{Z}/p^{n'}\mathbb{Z}\big)^{\oplus g}\rtimes \big(\mathbb{Z}/p^n\mathbb{Z}\big)^{\times}\ $ where g > 0 and
$0 \lt n'\leq n$. Given a modular Galois representation
$\overline{\rho}:G_{\mathbb{Q}}\rightarrow \text{GL}_2(\mathbb{F})$ which is p-ordinary and also p-distinguished, we shall write
$\mathcal{H}(\overline{\rho})$ for the associated Hida family. Using Greenberg’s notion of Selmer atoms, we prove an exact formula for the algebraic λ-invariant
\begin{equation}
\lambda^{\text{alg}}_{\mathcal{D}_n}(f) \;=\; \text{the number of zeroes of }
\text{char}_{\Lambda}\big(\text{Sel}_{\mathcal{D}_n^{\text{cy}}}\big(f\big)^{\wedge}\big)
\end{equation}
$f\in\mathcal{H}(\overline{\rho})$, under the assumption
$\mu^{\text{alg}}_{K(\mu_p)}(f_0)=0$ for at least one form f0. We can then easily deduce that
$\lambda^{\text{alg}}_{\mathcal{D}_n}(f)$ is constant along branches of
$\mathcal{H}(\overline{\rho})$, generalising a theorem of Emerton, Pollack and Weston for
$\lambda^{\text{alg}}_{\mathbb{Q}(\mu_{p})}(f)$.
For example, if $\mathcal{D}_{\infty}=\bigcup_{n\geq 1}\mathcal{D}_n$ has the structure of a p-adic Lie extension then our formulae include the cases where: either (i)
$\mathcal{D}_{\infty}/K$ is a g-fold false Tate tower, or (ii)
$\text{Gal}\big(\mathcal{D}_{\infty}/K(\mu_p)\big)$ has dimension ≤ 3 and is a pro-p-group.