וְהָיָ֗ה הַבְּכוֹר֙ אֲשֶׁ֣ר תֵּלֵ֔ד יָק֕וּם עַל־שֵׁ֥ם אָחִ֖יו הַמֵּ֑ת וְלֹֽא־יִמָּחֶ֥ה שְׁמ֖וֹ מִיִּשְׂרָאֵֽל׃
And it shall be, that the first-born that she beareth shall succeed in the name of his brother that is dead, that his name be not blotted out of Israel.
Read kabbalistic (mystical) interpretation of Deuteronomy 25:6: and classical mystical sources.