We deduce a formula enumerating the isomorphism classes of ex-
tensions of a
p
-adic field
K
with given ramification
e
and inertia
f
.
The formula follows from a simple group-theoretic lemma, plus the
Krasner formula and an elementary class field theory computation.
It shows that the number of classes only depends on the ramifica-
tion and inertia of the extensions
K
/
Q
p
,and
K
(ζ
p
m
)/
K
obtained
adding the
p
m
-th roots of 1, for all
p
m
dividing
e