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Enumerative combinatorics, graph theory, order theory, posets, matroids, designs and other discrete structures. It also includes algebraic, analytic and probabilistic combinatorics.
5
votes
Accepted
Sequence of permutations without a fixed point
Of course such permutations exist if $n > m$ (just take each $A_i$ to be the same $n$-cycle). On the other hand if $m \le n$, a pigeonhole argument should give some pair of $A_1, A_1 A_2, A_1A_2A_3, …
5
votes
Accepted
Hamming distance between $a+b$ and $a \oplus b \oplus ((a \land b) \ll 1)$
The lim sup is exactly $1$. Almost certainly the exact value of $D_n$ comes from $a = (1,1,1,\ldots,1)$ and $b = (1,0,0,\ldots,0)$, and even if not, it’s off by at most $O(1/n)$ which is inconsequent …