恒等写像
↑ (Knapp 2006)
↑ (松坂 1968, p. 28)
↑ (ブルバキ 1984, p. 10)
↑ (Marshal, Odell & Starbird 2007)
↑ (Anton 2005)
↑ (Shores 2007)
↑ (Anderson 2005)