# Questions tagged [discrete-mathematics]

The study of discrete mathematical structures. Consider using a more specific tag instead, such as: (combinatorics), (graph-theory), (computer-science), (probability), (elementary-set-theory), (induction), (recurrence-relations), etc.

22,055 questions
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### Find number of different strings that can are formed from {a,b,c}

I need to find the number of different strings that need to be formed from {a,b,c} in which there needs to be at least one from each letter. The question is to find the number of strings with length 5....
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### Solve recurrence with characteristic polynomial

The equation $$a_n=7\cdot a_{n-1} -7\cdot a_{n-2}+175\cdot a_{n-3}+450\cdot a_{n-4}+(5+13\cdot n)\cdot9^n \enspace,$$ where $a_0=148, a_1=144, a_2=-55, a_3=-61$. I assume that a solution will look ...
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### A string of odd and even numbers to group in three sets using combinatorics?

I need help with combinatorics problem. The task is this: There are 9 numbers which are: 1,3,5,2,4,6,8,10,12. I need to group these numbers in 3 sets with 3 elements in every set, but there is one ...
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### Cartesian product of R2 x R2 [on hold]

I need to work with a relation on $\mathbb{R}^2$ defined by $(x_1,y_1)R(x_2,y_2)$ with some condition. So far I've only done relations and Cartesian Products with single-dimension data (e.g., a ...
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### What units is my mean squared error if I center and scale my training data?

I have a KNN model that I used to predict the close price on houses. ...