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Problem 4 (30): Prove that the following languages are not regular. \text { (a) } L_{1}=\left\{a^{n} b^{n} c^{n} \mid n \geq 0\right\} \text { (b) } L_{2}=\left\{w w^{R} \mid w,

w^{R} \in\{a, b\}^{*} \text { and } w^{R} \text { is the reversal of } w\right\} \text { (c) } L_{4}=\left\{w \mid w \in\{a, b\}^{*} \text { and the number of } a \text { 's is greater than the mumber of } b\right. \text { 's\} }

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