Cyclic Vanishing Identities of Sun-Pan Type: Analytic and Modular Perspectives

We revisit the cyclic identities of Sun--Pan type for Bernoulli polynomials and their $q$-analogues. From the analytic side, we formulate minimal Appell axioms that force cyclic vanishing identities, extending naturally to $q$-Appell sequences and analytic Bernoulli functions. From the modular side, we show that the same relations arise as period polynomial identities associated with Eisenstein series, reflecting the symmetry $(ST)^3=-I$ of the modular group. These two complementary perspectives place the Sun--Pan cyclic identities at the crossroads of number theory, special functions, and modular forms, and suggest further connections to polylogarithms, $L$-values, and mixed Tate motives.