Forbidden conductors and sequences of $\pm 1$s

We study “forbidden” conductors, i.e. numbers q > 0 satisfying algebraic criteria introduced by J. Kaczorowski, A. Perelli and M. Radziejewski [Acta Arith. 210 (2023), 1-21], that cannot be conductors of L-functions of degree 2 from the extended Selberg class. We show that the set of forbidden q is dense in the interval (0,4), solving a problem posed in [Acta Arith. 210 (2023), 1-21]. We also find positive points of accumulation of rational forbidden q.