2018 USAJMO Problems/Problem 6
Problem 6
Karl starts with cards labeled
lined up in a random order on his desk. He calls a pair
of these cards swapped if
and the card labeled
is to the left of the card labeled
. For instance, in the sequence of cards
, there are three swapped pairs of cards,
,
, and
.
He picks up the card labeled 1 and inserts it back into the sequence in the opposite position: if the card labeled 1 had card to its left, then it now has
cards to its right. He then picks up the card labeled
and reinserts it in the same manner, and so on until he has picked up and put back each of the cards
exactly once in that order. (For example, the process starting at
would be
.)
Show that, no matter what lineup of cards Karl started with, his final lineup has the same number of swapped pairs as the starting lineup.
Solution
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.
See also
2018 USAJMO (Problems • Resources) | ||
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