Genus-zero and genus-one string amplitudes and special multiple zeta values

In this paper we show that in perturbative string theory the genus-one contribution to formal 2-point amplitudes can be related to the genus-zero contribution to 4-point amplitudes. This is achieved by studying special linear combinations of multiple zeta values that appear as coefficients of the amplitudes. We also exploit our results to relate closed strings to open strings at genus one using Browns single-valued projection, proving a conjecture of Broedel, Schlotterer and the second author.