DOCS.
486,
487
MARCH
1918 503
professors
is
dated
20
Nov[em]b[e]r
1913
and bears the
file
number
U. I. K. No.
4054.[8]
With that
you
will
easily
steer Mr.
Kruss
on
to
the
right
track. If in
the
course
of
the
negotiations you
should believe
that
you may
in
any way
be in need
of
my involvement,
I
naturally
am
always
at
your disposal.
With
cordial
regards,
yours,
Planck.
487. From
Felix Klein
Göttingen,
20
March
1918
Esteemed
Colleague,
Here
is
my
long
reply
now
to
your
v[alued]
letter
of
the
13th.[1]
First
of
all,
my agreement
with
your closing
remark:
I
had noticed at
the
t[ime],
during
proofing,
that
my
statement
on
p.
13 of
my
note[2]
that the
special
theory
has
10 field
equations
Kuv
=
0
could
seem
misleading
(because
there
are
the
20
equations
(uv,
pa)
=
0,
of
course),
but
had left it because
the
situation is
described
clearly
at
the bottom
of
p. 5.[3]
Then:
on page
9
of
my
note,
l[ines]
3,
4
from
above,
the
/g
(which was
deleted
in
the
separatum
sent
to
you)
must be
retained
after
all[4] and,
as
I
now see, a
minus
sign
must be inserted before
the
summation
signs.-
In
other
respects,
however,
I want to
stand
by
the considerations
of No. 9
of
my
note[5]
and
substantiate
them here in
that
I
compare
them
formula
by
formula
to
your paper
on
Hamilton’s
principle, etc.,
in
the Berliner
Sitzungsberichte
of
1916:[6]
1)
My
K's
and
aQ's,
multiplied by
/g,
are
directly your
G’s
and
M’s,
insofar
as
I
disregard
the
special
form
of
Q,[7]
to which
I
restrict
myself
in
my
note
but
which
are
used
only
in
the
definition
of
6qp
in
f[ormula]
(13)[8]
and in
the
transition
from
(14)
to
(14').
2)
My Lagrangian
derivatives,
Kuv,
aQuv,
again
multiplied by
/g,
read for
you
as:
dG*
_
_d_
f
dG*
\
dM
dg^u
dxa \dgliu) ’
dg^v
For
the
Lagrangian derivatives,
there
is
indeed
no
difference in
operating
with
G
or
with
G*.
3)
Instead
of
Kuv,
aQuv,
I
can now,
in order
to
come
closer to
your designation,
introduce
“mixed”
components
as
needed:
K
=
j9-Y,Kv,gr
aQ:
=
a^g-J2Q^.