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Zassenhaus formula:
e
t
(
X
+
Y
)
=
e
t
X
e
t
Y
e
−
t
2
2
[
X
,
Y
]
e
t
3
6
(
2
[
Y
,
[
X
,
Y
]
]
+
[
X
,
[
X
,
Y
]
]
)
e
−
t
4
24
(
[
[
[
X
,
Y
]
,
X
]
,
X
]
+
3
[
[
X
,
Y
]
,
X
]
,
Y
]
+
3
[
[
[
X
,
Y
]
,
Y
]
,
Y
]
)
…
Related:
Baker-Campbell-Hausdorff formula
and
flow of the sum of two vector fields
.