Olympiad Combinatorics

weight independent set.
12. [IMO Shortlist 2013, C3] 
A crazy physicist discovered a new kind of particle which he 
called an imon. Some pairs of imons in the lab can be 
entangled, and each imon can participate in many 
entanglement relations. The physicist has found a way to 
perform the following two kinds of operations with these 
particles, one operation at a time.
(i) If some imon is entangled with an odd number of other 
imons in the lab, then the physicist can destroy it. 
(ii) At any moment, he may double the whole family of imons 
in the lab by creating a copy I’ of each imon I. During this 
procedure, the two copies I’ and J’ become entangled if and 

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Olympiad Combinatorics 
26 
only if the original imons I and J are entangled, and each 
copy I’ becomes entangled with its original imon I; no 
other entanglements occur or disappear at this moment. 
Show that after a finite number of operations, he can ensure 
that no pair of particles is entangled. 
13. [Japan 1998] 
Let n be a positive integer. At each of 2n points around a circle 
we place a disk with one white side and one black side. We 
may perform the following move: select a black disk, and flip 
over its two neighbors. Find all initial configurations from 
which some sequence of such moves leads to a position where 
all disks but one are white. 
14. [Based on IOI 2007] 
You are given n integers a
1
, a
2
, …, a
n
and another set of n 
integers b
1
, b
2
, …, b
n
such that for each i, b
i
≤ a
i
. For each i = 1, 
2, …, n, you must choose a set of b
i
distinct integers from the 
set {1, 2, …, a
i
}. In total, (b
1
+ b
2
+…+ b
n
) integers are selected, 
but not all of these are distinct. Suppose k distinct integers 
have been selected, with multiplicities c
1
, c
2
, c
3
, …, c
k
. Your