1
$\begingroup$

I don't know Swedish and I'm not finding the article "G. Blom and C. E. Froberg, On money changing" translated into English... so I tried to read the original (Swedish) with the help of Google Translate but, from the results, I fear that some semantics could get lost...
The article prove a denumerant's upper and lower bound:
$\frac{n^{k-1}}{(k-1)!\prod_{i=1}^{k}a_i} \leq d(n;a_1,\dots,a_k) \leq \frac{(n+s_k)^{k-1}}{(k-1)!\prod_{i=1}^{k}a_i}$
where
$s_1=1, s_2=a_2$ and $s_k=a_2+\frac{1}{2}(a_3+\dots+a_k), \forall k\geq 3$.
I would like to know: is it required (hypothesis) that all $a_i$ must be coprime i.e. $(a_1,\dots,a_k)=1$?
I first read about this theorem in the book "J. L. R. Alfonsin, The Diophantine Frobenius Problem" (pag. 74) but what I'm asking is not specified.

UPDATE
I would have written in the comments but it was too long.
Despite @Carl-FredrikNybergBrodda's competent reassurance about the translation (i.e. I'm assuming that there are no explicit statements about co-primality) something in the back of my mind was not in peace... unfortunately very often there are implicit hypotheses in math articles (especially dated ones).
The reasons why this happen... well... are different but it is usually because are considered trivial by the context (or by the author).
So yesterday I started reading the mathematical component of the article trying to reconstruct the proof and from what I seemed to understand... is that the implicit hypothesis is stronger than $(a_1,...,a_k)=1$ i.e. it's $a_1=1$.
This idea is supported by these observations:

  1. This article is about coins and there is always the currency of unitary value (at least $"a.s."$).
  2. All the explicit example start with $a_1=1$
  3. pag. 60 the proof is by induction so there are two "base case" because the general expression for $s_i$ does not cover $s_1$ (by the way in the article the sequence $s$ is called $\Delta$), therefore it is necessary to check $m=1$ and $m=2$ before the inductive step $m\geq 3$.
    The case $m=2$ check the truthfulness of:
    $\frac{1}{(2-1)!}\frac{n^{2-1}}{a_2}\leq D(2,n)\leq \frac{1}{(2-1)!}\frac{(n+s_2)^{2-1}}{a_2}$
    but if it was $a_1\neq 1$ should be:
    $\frac{1}{(2-1)!}\frac{n^{2-1}}{a_1 a_2}\leq D(2,n)\leq \frac{1}{(2-1)!}\frac{(n+s_2)^{2-1}}{a_1 a_2}$

Maybe I didn't understand the proof or maybe the case $a_1 = 1$ is not restrictive (but I don't see why).

$\endgroup$
4
  • $\begingroup$ Do you have a pdf of the original Swedish version? I cannot find it online. I can translate the relevant parts for you. $\endgroup$ Commented Jul 10, 2020 at 18:16
  • $\begingroup$ Yes, I have it! in advance a giant THANK YOU! I am new to the forum how can I send you the article? $\endgroup$
    – Ramanumpy
    Commented Jul 10, 2020 at 18:25
  • $\begingroup$ If you have a link to it, posting it here is probably easiest. Otherwise, you can send it to me via the email on my profile. $\endgroup$ Commented Jul 10, 2020 at 18:27
  • $\begingroup$ I just sent you an email with the article. Thank you very much for helping. $\endgroup$
    – Ramanumpy
    Commented Jul 10, 2020 at 19:21

1 Answer 1

1
$\begingroup$

The article is [Blom, G. and Fröberg, C-E., Om Myntväxling, Nordisk Matematisk Tidskrift, 1962, Vol. 10, No. 1/2 (1962), pp. 55-69] for anyone who wishes to sing along.

After reading through the article, no assumption is made on the coprimality of the $a_i$, in the sense that no added assumptions appear to be stated on the $a_i$ before the statement of the theorem.

As an aside, the Swedish used in this article is absolutely gorgeous, and is well worth the read. My only concern is that the authors assume that whoever is reading the article knows the Swedish currency system well, but not everyone is aware that there are $100$ öre in $1$ krona... to further complicate matters for modern readers, the öre (equivalent to a cent) is no longer in use in Sweden.

$\endgroup$
6
  • $\begingroup$ Unfortunately it was precisely the examples that made me suspicious because they are all coprime cases due to the first value 1 (common to all the examples). The lack of explicit declaration of coprimality and the observation you make on page 57 I would say that they close the discussion making an even more remarkable a beautiful theorem. Thanks. $\endgroup$
    – Ramanumpy
    Commented Jul 10, 2020 at 20:10
  • $\begingroup$ "their many examples clearly show that they do not assume this" - do you mean they do assume this by any chance? $\endgroup$ Commented Jul 11, 2020 at 2:16
  • $\begingroup$ @MaxAlekseyev No, I mean it as it is written; "their many examples clearly show that they do not assume [coprimality of the $a_i$]". $\endgroup$ Commented Jul 11, 2020 at 7:43
  • $\begingroup$ I'm confused then. Your bullet points do not support this conclusion (only that they assume no pairwise coprimality), and according to @Ramanumpy, all their examples contain 1 and thus are co-prime. $\endgroup$ Commented Jul 11, 2020 at 12:20
  • 1
    $\begingroup$ They use $d_m$ as the GCD of $a_1,\dotsc,a_m$. One can then quite easily factor out this from the problem, and assume $a_1=1$ without losing much. $\endgroup$ Commented Jul 11, 2020 at 21:16

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .