Numbering related equations with subindexes (2.1, 2.2 etc.)

[Micha Feigin]

I have some related equations that I would like to number 2.1, 2.2 etc. instead
of 2, 3 as the other equation numbers run. Is this possible?

i.e something like

a = b (1)

blah blah

c = b (2.1)
c = d (2.2)

some more blah

e = mc^2 (3)


Thanks