
Well, finally BNCG is published now. You can check its cover here. The book is published by the European Mathematical Society Publishing House.
Happy holidays to everyone!
Upon expanding out via the binomial theorem, and summing over , we find
}
where is the set of positive integers divisible by .
Let be the zeroes of ; it is obviously closed. When , Warren (in his paper and in personal communication) showed that consists pricisely of those non-negative integers such that the sum of the -adic digits of is less than . such that the reduction of is nonzero. □
There are other important results that arise from the first part of the
Lemma. Indeed, upon replacing with , we obtain
}
This immediately gives the action of on the Mahler expansion of
a continuous function from to characteristic . One also obviously has
}
\sum_k {\sigma(y) \choose \sigma (k)}x^{\sigma(k)\,.\sum {y \choose k}x^{\sigma k}\,,
But, by the first part of the Lemma, this then equals }\sigma (z^i/i!):= z^{\sigma (i)}/\sigma(i)! \.
which is a sort of change of variable formula. As the action of is continuous on there is a dual action on measures; if the measures are characteristic valued, then this action is easy to compute from (*) above. However, there is ALSO a highly mysterious action of on the *convolution algebra* of characteristic valued measures on the maximal compact subrings in the completions of at its places of degree (e.g, the place at or associated to , if the place has higher degree one replaces with the appropriate subgroup). Indeed, given a Banach basis for the space of -linear continuous functions from that local ring to itself, the "digit expansion principle" gives a basis for ALL continuous functions of the ring to itself (see, e.g., Keith Conrad, "The Digit Principle", J. Number Theory 84 (2000) 230-257). In the 1980's Greg Anderson and I realized that this gives an isomorphism of the associated convolution algebra of measures with the ring of formal *divided power series* over the local ring. But let and define }
The content of the second part of the Lemma is precisely that this definition gives rise to an algebra automorphism of the ring of formal divided power series.
which is an element of . Let be the valuation of at the place of .
Dinesh's "main recursion formula" then states that:
}
This then leads iteratively to the second recursion formula
}
where means the -composition of the map with itself.
The main recursion formula is highly remarkable in that one computes a sum over the monics of degree and then finds its valuation at and *then* uses this integer as the exponent to raise the monics of degree . This feedback loop is absolutely new in terms of anything that I have ever seen.
One can ask whether there are any classical analogs of the above recursion formulas. It may be that when things are much better known, the second recursion formula will be viewed as the -analog of the basic formula
}