# CMI2020-A: 5

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A boolean function on $n$ variables is a function $f$ that takes an n-tuple of boolean values $x \in \{0,1\}^n$ as input and produces a boolean value $f(x)\in \{0,1\}$ as output.

We say that a boolean function $f$ is symmetric if, for all inputs $x,y \in \{0,1\}^n$ with the same number of zeros (and hence the same number of ones), $f(x)=f(y)$. What is the number of symmetric boolean functions on $n$ variables?

1. $n+1$
2. $n!$
3. $\displaystyle \sum^n_{i=0} \begin{pmatrix} n\\i \end{pmatrix}$
4. $2^{n+1}$
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Let $\Sigma=\{a,b\}.$ For two non-empty languages $L_1$ and $L_2$ over $\Sigma$, we define $Mix(L_1,L_2)$ to be $\{w_1\;u\;w_2\;v\;w_3|\;u\in L_1,v\in L_2,w_1,w_2,w_3\in \Sigma^*\}$. Give two languages $L_1$ and $L_2$ ... $Mix(L_1,L_2)$ is also regular. Provide languages $L_1$ and $L_2$ that are not regular, for which $Mix(L_1,L_2)$ is regular.
Which of the following languages over the alphabet $\{0,1\}$ are $not$ recognized by deterministic finite state automata $(DFA)$ with $three$ states? Words which do not have $11$ as a contiguous subword Binary representations of multiples of three Words that have $11$ as a suffix Words that do not contain $101$ as a contiguous subword
Consider the following regular expressions over alphabet$\{a,b\}$, where the notation $(a+b)^+$ means $(a+b)(a+b)^*$: $r_1=(a+b)^+a(a+b)^*$ $r_2=(a+b)^*b(a+b)^+$ Let $L_1$ and $L_2$ be the languages defined by $r_1$ and $r_2$, respectively. Which of the following regular expressions define $L_1\cap L_2$ ... $(a+b)^*a\;b(a+b)^*$ $(a+b)^*b(a+b)^*a(a+b)^*$ $(a+b)^*a(a+b)^*b(a+b)^*$