Reference for Yevamot 44:1

מיזקק נמי זקיק:

he also has the power to impose the obligation of the levirate marriage.<span class="x" onmousemove="('comment',' Upon the wife of any son of his father. However, since he is debarred from marrying her, he frees her by halizah, v. supra 20a. ');">¹</span>

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