$n(n+1)/2$ coins are placed on an equilateral lattice with side $n$, such that all but one coin are showing heads.

At each step, one is allowed to choose two adjacent coins $A$ and $B$, and then flip all coins on the line $AB$ (and its extension).

Characterize all starting configuration such that it's always possible to get all tails.

## Tuesday, March 1, 2011

### Coins in equilateral lattice

Labels:
coin,
Combinatorics,
equilateral lattice,
flip,
invariance,
lattice,
steps

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Bagus juga soalnya. :D Kuncinya tinjau tiga koin di ujung. Setiap langkah pasti membalik 0 atau 2 di antaranya.

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