人為規(guī)定的.
觀察通項(xiàng)23n+6可知23,26,29不滿(mǎn)足通項(xiàng),為什么?
就因?yàn)轫?xiàng)數(shù)為n+3
例：n=1時(shí),項(xiàng)數(shù)為4,末項(xiàng)為23*1+6=29
數(shù)列即為1,23,26,29
n=2時(shí),項(xiàng)數(shù)為5,末項(xiàng)為23*2+6=52
看懂了嗎,規(guī)定項(xiàng)數(shù)為n+3保證了該有窮數(shù)列是前三項(xiàng)為1,23,26從第四項(xiàng)起為公差23的等差數(shù)列.若定義小于3（例n+2）,則不能滿(mǎn)足前4項(xiàng)（已知的四項(xiàng)）.若定義大于3（例n+4）則數(shù)列項(xiàng)數(shù)不夠.以n+4例：n取1,則有5項(xiàng),而末項(xiàng)為23*1+6=29,即1,23,26,29只有四項(xiàng)