There is a rather difficult formula (simplified in 1973). The subsequent published value 6222928 for n=6 needs to be verified.
Table of n, a(n) for n=1..5.
V. A. Liskovets, The number of strongly connected directed graphs, Mat. Notes, 8 (1970), 877-882
Sequence in context: A213403 A077281 A239774 * A222672 A024295 A291658
Adjacent sequences: A049384 A049385 A049386 * A049388 A049389 A049390
Valery A. Liskovets