> As you say, "..." produces centred dots in eqn. Groff's eqn (1.18.1) > does not recognise the token 'cdots', and 'man eqn' does not mention > it.
This has been changed in 1.20 (and discussed on the mailing list); to
cite the man page:
GNU eqn sets the input token "..." as three periods or low dots,
rather than the three centered dots of classic eqn. To get three
centered dots, write `cdots' or `cdot cdot cdot'.
Werner
