#### prerequisites: The post A dual to Moessner’s sieve

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Two chords are pushing it.
Three chords and you’re into jazz.”
Lou Reed

### 1. Introduction

The goal of this blog post is to state Moessner’s idealized theorem, Long’s idealized theorem, and conjecture a further generalization.

The chapter is structured as follows. In Section 2 we start by adapting Moessner’s theorem to the dual sieve in order to obtain Moessner’s idealized theorem. Then, in Section 3, we repeat the process for Long’s theorem, which motivates the statement of Long’s idealized theorem along with Long’s weak theorem. Finally, we conjecture a further generalization of Long’s idealized theorem in Section 4. The post is concluded in Section 5.

### 2. Moessner’s idealized theorem

In order to obtain Moessner’s idealized theorem, we start from Moessner’s original theorem, generalize it and adapt it to the dual sieve.

Theorem 1 (Moessner’s theorem). Given an initial sequence of positive natural numbers,

and a natural number $k \ge 2$, we obtain the result sequence of successive powers,

when performing the following procedure:

1. Drop every $k$th element of the inital sequence,
2. partially sum the remaining elements into a new sequence, and
3. decrease $k$ by $1$.

The above procedure is repeated if $k>1$ and stops if $k=1$. We call the procedure above Moessner’s sieve and call $k$ the rank of the sieve.

As we saw in the first post on Moessner’s theorem we can generalize the initial sequence and still obtain the same result sequence,

Theorem 2 (Moessner’s generalized theorem). Given the initial sequence of $1$ followed by $0$s,

and a natural number $k + 2$, we obtain the result sequence of successive powers,

when applying Moessner’s sieve on the initial sequence.

So, if we let the rank $k = 4$, an example application of Theorem 2 becomes,

where we still obtain a result sequence of values to the fourth power. If we compare the above example to where we left off in the post on the dual of Moessner’s sieve,

where we introduced the concept of an initial configuration of a sieve along with the concept of seed tuples, we notice that instead of an initial sequence of $1$ followed by $0$s, we have an initial configuration of two seed tuples, where the horizontal seed tuple is filled with $0$s and the vertical seed tuple is filled with a $1$ followed by $0$s. Likewise, instead of a result sequence of values to the forth power, we obtain a sequence of Moessner triangles, whose bottom-most elements enumerate the same result sequence of values to the forth power. If we apply these differences to the statement of Theorem 2, we obtain Moessner’s idealized theorem,

Theorem 3 (Moessner’s idealized theorem). Given an initial configuration of two seed tuples of length $k + 2$,

we obtain the sequence of Moessner triangles of rank $k$, where the bottom-most elements enumerate the sequence,

Having gone from Moessner’s original theorem to Moessner’s idealized theorem, which reflects the structure of the dual sieve, we proceed to apply similar transformations to Long’s theorem.

### 3. Long’s idealized theorem

In order to state Long’s idealized theorem, we follow the same process as established in the previous section, and repeat the traditional definition of Long’s theorem and afterwards adapt it to the dual sieve.

Theorem 4 (Long’s theorem). Given an initial sequence, which can be described as an arithmetic progression,

we obtain the result sequence,

when applying Moessner’s sieve of rank $k$ on the initial sequence.

We can visualize Long’s theorem as the sieve,

for which Long1 also noted that we can generalize the sieve by adding a row of $d$s,

However, this unfortunately gives us an inconsistent initial column, since it contains a $d$ at the top but $c$s in the remaining entries. So, we take the liberty of adjusting the initial configuration of the sieve to better suit our dual sieve, by ridding ourselves of the above inconsistency. Thus, we move the $c$s of the initial column into the vertical seed tuple, and at the same time generalize to a single seed value $c$, while putting the $d$s into the horizontal seed tuples, yielding the following sieve,2

While moving the $c$s has changed the coefficients of the $d$s in the sieve, we now have a more consistent initial configuration, which we believe to be in the spirit of Long’s original theorem, with one constant, $c$, in the vertical seed tuple and the horizontal seed tuples filled with the constant $d$. As it turns out, we can perform a further generalization of the initial configuration by replacing the sequence of $d$s with a $d$ followed by $0$s, while putting it in the vertical seed tuple, as we did with the sequence of $1$s when we defined the dual of Moessner’s sieve,

This results in a minimal initial configuration consisting of a vertical seed tuple containing a constant $d$ and a constant $c$ followed by $0$s. We can now state Long’s idealized theorem as follows,

Theorem 5 (Long’s idealized theorem). Given an initial configuration of two seed tuples of length $k + 2$,

we obtain the sequence of Moessner triangles of rank $k$, where the bottom-most elements enumerate the sequence,

for values of $t \ge 0$, when applying the dual of Moessner’s sieve on the initial configuration.

As a result of the transformations made above, we now notice that the coefficients of the $c$s correspond to the values of Moessner triangles at rank $k$ while the coefficients of the $d$s now correspond to the values of Moessner triangles at rank $k - 1$. This observation suggests that we can view the above sieve as the composition of two sieves, one creating Moessner triangles of rank $3$ filled with $c$s,

and one creating Moessner triangles of rank $4$ filled with $d$s,

This suggests that there is an additional step between Moessner’s idealized theorem and Long’s idealized theorem, where we generalize the seed value of $1$ in Moessner’s idealized theorem to a constant, $c$, and obtain Long’s weak theorem,

Theorem 6 (Long’s weak theorem). Given an initial configuration of two seed tuples of length $k + 2$,

we obtain the sequence of Moessner triangles of rank $k$, where the bottom-most elements enumerate the sequence,

for values of $t \ge 0$, when applying the dual of Moessner’s sieve on the initial configuration.

Having defined Long’s idealized theorem and Long’s weak theorem, we try to look beyond Long’s theorem in the next section.

### 4. Beyond Long’s idealized theorem

Since Long’s idealized theorem describes the result sequence generated by Moessner’s sieve, when starting from a seed tuple of two constants, $c$ and $d$,

we now ask the obvious question of what happens if we start from a seed tuple of $3$ or even $n$ values? Looking at the result sequence of the above sieve, we know that it enumerates the values of the binomial, $c \cdot {(1+t)}^3 + d \cdot {(1+t)}^4$, which gives us the idea to label $c = a_3$ and $d = a_4$, and fill the rest of the seed tuple with $a_i$,

yielding the above Moessner triangle. Now, if we examine the entries of the hypotenuse in this triangle,

we notice that we can rearrange them into the following Pascal-like triangle,

where the sum of the entries yields the following result,

located at the bottom of the first column of the second triangle of the sieve, which we can restate as,

Likewise, if we calculated the next triangle and the subsequent first column, we would obtain the values,

which we can once again restate as,

This observation suggests that the application of Moessner’s sieve on an initial configuration with the vertical seed tuple,

yields a sequence of Moessner triangles whose bottom-most elements enumerate the values of the polynomial,

where $t$ is the triangle index. Thus, we conjecture that applying the dual sieve on an initial configuration where the vertical seed tuple consists of the constants,

yields a sequence of Moessner triangles where the bottom-most elements, comprising the result sequence, enumerate the values of the polynomial,

The reason why the above statement is a conjecture and not a theorem is because the previous theorems have all been formalized and proved in my Master’s thesis, while this last statement has not. However, there are strong indications of its correctness as we can decompose a seed tuple consisting of any sum of two tuples and have proved that the conjecture holds for the binomial $a_{i+1} \cdot (1 + t)^{i+1} + a_i \cdot (1 + t)^i$, as stated by Long’s idealized theorem.

### 5. Conclusion

In this blog post, we have introduced idealized versions of Moessner’s theorem and Long’s theorem – along with Long’s weak theorem – stated in terms of the dual of Moessner’s sieve. Furthermore, we have conjectured a new generalization of Long’s theorem that connects it to polynomial evaluation.

This post is the final excerpt from my Master’s thesis, in which I formalize and prove all the statements mentioned in this and the previous posts in the Coq proof assistant.

1. See “On the Moessner Theorem on Integral Powers” (1966) by Calvin T. Long.

2. The addition of the dotted vertical lines between each column is for the sake of readability.