What's the function of the word "ли" in the following contexts? I came across of this word in many sentences in a conversation book, and I really don't understand it and where I have to put it. Is there equivalent in English?

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> Есть **ли** у вас дети? (Have you got any children?)
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> Могу **ли** я пригласить вас на обед? (May I invite you to lunch?)
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