It's not forever because it terminates after 2T periods of six-month interval each.

债券不是永续的,券息每六个月为一期,共支付2T期

耶鲁公开课 - 金融市场课程节选

You want to subtract off the value of a perpetuity that starts after 2T, six-month intervals, so this is the present value of the perpetuity that starts after 2T, six-month intervals.

所以就要从永续债券价格里减去,2T期以后,每六月一期券息的贴现值,这部分是永久债券从第2T期后,每半年一期券息的现值

耶鲁公开课 - 金融市场课程节选