Difference between recursive language and recursively enumerable language by 
Ciro Santilli 40 Updated 2025-07-16
Ciro Santilli 40 Updated 2025-07-16This section is about the definition of the dot product over , which extends the definition of the dot product over .
Some motivation is discussed at: math.stackexchange.com/questions/2459814/what-is-the-dot-product-of-complex-vectors/4300169#4300169
Just like the usual dot product, this will be a positive definite symmetric bilinear form by definition.
Computational problem where the solution is either yes or no.
When there are more than two possible answers, it is called a function problem.
Decision problems come up often in computer science because many important problems are often stated in terms of "decide if a given string belongs to given formal language".
The canonical undecidable problem.
Of course, because what we know about the halting problem, there cannot exist a single decider that decides all Turing machines.
E.g. The Busy Beaver Challenge has a set of deciders clearly published, which decide a large part of BB(5). Their proposed deciders are listed at: discuss.bbchallenge.org/c/deciders/5 and actually applied ones at: bbchallenge.org.
But there are deciders that can decide large classes of turing machines.
Many (all/most?) deciders are based on simulation of machines with arbitrary cutoff hyperparameters, e.g. the cutoff space/time of a Turing machine cycler decider.
The simplest and most obvious example is the Turing machine cycler decider
Bibliography: discuss.bbchallenge.org/t/decider-cyclers/33
Example: bbchallenge.org/279081.
These are very simple, they just check for exact state repetitions, which obviously imply that they will run forever.
Unfortunately, cyclers may need to run through an initial setup phase before reaching the initial cycle point, which is not very elegant.
Described at: www.sligocki.com/2022/06/10/ctl.html
The busy beaver game consists in finding, for a given , the turing machine with states that writes the largest possible number of 1's on a tape initially filled with 0's. In other words, computing the busy beaver function for a given .
There are only finitely many Turing machines with states, so we are certain that there exists such a maximum. Computing the Busy beaver function for a given then comes down to solving the halting problem for every single machine with states.
Some variant definitions define it as the number of time steps taken by the machine instead. Wikipedia talks about their relationship, but no patience right now.
The Busy Beaver problem is cool because it puts the halting problem in a more precise numerical light, e.g.:
- the Busy beaver function is the most obvious uncomputable function one can come up with starting from the halting problem
- the Busy beaver scale allows us to gauge the difficulty of proving certain (yet unproven!) mathematical conjectures
The following things come to mind when you look into research in this area, especially the search for BB(5) which was hard but doable:
- it is largely recreational mathematics, i.e. done by non-professionals, a bit like the aperiodic tiling. Humbly, they tend to call their results lemmas
- complex structure emerges from simple rules, leading to a complex classification with a few edge cases, much like the classification of finite simple groups
Bibliography:
Pinned article: Introduction to the OurBigBook Project
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