EN
A congruence of Emma Lehmer (1938) for Euler numbers $E_{p-3}$ modulo p in terms of a certain sum of reciprocals of squares of integers was recently extended to prime power moduli by T. Cai et al. We generalize this further to arbitrary composite moduli n and characterize those n for which the sum in question vanishes modulo n (or modulo n/3 when 3|n). Primes for which $E_{p-3} ≡ 0 (mod p)$ play an important role, and we present some numerical results.