We need to fill each cell in a grid with a distinct integer from 1 to 36. There are two rules:
Over all valid configurations, how many of the 36 cells could contain the number
Note
: Below is a
grid adhering to the rules above.
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We will prove that 3 6 can only be in a corner.
It's easy to see that every even number will be placed on the same colour. We have nine numbers from
1
to
3
6
which have the
4
divisor:
4
,
8
,
1
2
,
1
6
,
2
0
,
2
4
,
2
8
,
3
2
,
3
6
. We can suppose that even numbers will be placed on the red cells. In the figure we dismembered the table to nine pieces of
2
×
2
square. There cannot be two-divisible by
4
-numbers in one
2
×
2
square, so in every
2
×
2
square there will be exactly one number which is divisible by
4
. The
6
×
6
grid has a rotational symmetry, so we can suppose that in the middle
2
×
2
square this number (which is divisble by
4
) is in the F cell. Then in the
A
,
B
,
C
,
E
,
G
,
I
,
J
,
K
cells there must not be a number, which is a multiply of
4
. So in the
I
J
M
N
2
×
2
square the number, which is divisible by
4
, can only be in cell
N
(because
M
is a blue cell). Similarly in the
C
D
G
H
2
×
2
square the only possible place for the number, which is divisble by
4
, is cell
H
. Now we can see that in the
K
L
O
P
2
×
2
square the number, which is a multiply of
4
, can't be in
K
,
L
,
O
cells, so it must be on the
P
cell. We will show that the only number, which can be on the
P
cell, is
3
6
. If it were different (for example
4
∗
a
) then
1
<
4
∗
a
<
3
7
would be true, so
4
a
+
2
and
4
a
−
2
would have to be placed on the grid. These numbers can't be on the
N
,
H
cells (because
4
is not a divisor of
4
a
−
2
and
4
a
+
2
)
, so in cell
K
there would have to be
2
numbers, which would not be possible. So
3
6
has to be in a corner, and there are
4
corners. A possible numbering: