Irregular weight-one Eisenstein series and Euler systems

Let f=E₁(χ₁,χ₂) be an irregular Eisenstein series of weight one, that is, with χ₁(p)=χ₂(p), and consider the generically cuspidal ordinary family passing through it. When χ₁, χ₂ and χ₁χ₂⁻¹ are primitive of the same conductor, we show that the specialization at f of the modular-symbol p-adic L-function of this family, with periods taken in the cohomology of the compactified modular curves, factors as a product of two Kubota--Leopoldt p-adic L-functions attached to χ₁ and χ₂. Since there are no complex critical values in weight one, the factorization is obtained directly from the cohomological construction: we compute the tame-twisted modular measure by means of the localized Sharifi map and explicit reciprocity, and show that tame twisting identifies the compact period modules at weight one. This gives a weight-one analogue of the factorization of Bellaïche--Dasgupta for critical-slope Eisenstein series. As applications, and under explicit specialization hypotheses, we compare Kato classes with circular units, Heegner classes with elliptic units, and diagonal cycles with Kato classes through two successive Eisenstein degenerations. We also discuss the passage from compact to open periods, and include an exposition of the congruence and degeneration methods underlying these comparisons.