[Retros] happy prime new year

[Noam Elkies]

I wrote:

>> This equivalence is not obvious, though I have not tried to
>> confirm or refute it.

andrew buchanan <andrew at anselan.com> replies:

> A string containing symbols 1..n can be regarded as a function from
> position i to value j (e.g. for 1342 1->1, 2->3, 3->4 & 4->2).
> But also it can be regarded as the inverse: [...]

Oh, right, a "sequential run" means j,j+1,j+2 in order, not an arbitrary
three numbers in order -- so yes, the inverse permutation gives the
bijection.

NDE