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Mathematics
Quick fact
Kruskal's tree theorem, first proved in 1960 by Joseph Kruskal, is so powerful that it is unprovable in Peano arithmetic, yet it can be proved in a stronger system of second-order arithmetic.
Why this is interesting
Imagine an infinite sequence of Christmas trees, each one no more complex than the next. Could you ever find a later tree that contains an earlier one as a pattern? The Kruskal tree theorem says yes—always!
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Subjects · study & training · careers
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