수학 얘기: 역사상이 존재한다. 즉, {\displaystyle f\circ g=\operatorname {id} _{Y}}<img width='0' height='0' class='mwe-math-fallback-image-inline image-lazy-loaded' alt='f\circ g=\operatorname {id}_{Y}' usemap='https://ko.m.wikipedia.org/wiki/undefined' src='https://wikimedia.org/api/rest_v1/media/math/render/svg/514196a322fb9ae59c4253888cc1350756465bc9' srcset='' style='margin: 0px; padding: 0px; border-style: initial; font-style: inherit; font-variant: inherit; font-weight: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: -0.671ex; background: none; display: inline-block; animation: 0.3s ease-in 0s 1 normal none running fadeInImage; width: 11.113ex; height: 2.509ex;'>, {\displaystyle g\circ f=\operatorname {id} _{X}}<img width='0' height='0' class='mwe-math-fallback-image-inline image-lazy-loaded' alt='g\circ f=\operatorname {id}_{X}' usemap='https://ko.m.wikipedia.org/wiki/undefined' src='https://wikimedia.org/api/rest_v1/media/math/render/svg/2935e82106a6aeb3c354d02246be16fe2ac7e4a0' srcset='' style='margin: 0px; padding: 0px; border-style: initial; font-style: inherit; font-variant: inherit; font-weight: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: -0.671ex; background: none; display: inline-block; animation: 0.3s ease-in 0s 1 normal none running fadeInImage; width: 11.259ex; height: 2.509ex;'>인 사상 {\displaystyle g\colon Y\to X}<img width='0' height='0' class='mwe-math-fallback-image-inline image-lazy-loaded' alt='g\colon Y\to X' usemap='https://ko.m.wikipedia.org/wiki/undefined' src='https://wikimedia.org/api/rest_v1/media/math/render/svg/263df9f4ac00972d999b70dafb0a2f485531fa7e' srcset='' style='margin: 0px; padding: 0px; border-style: initial; font-style: inherit; font-variant: inherit; font-weight: inherit; font-stretch: inherit; line-height: inherit; font-family: inherit; vertical-align: -0.671ex; background: none; display: inline-block; animation: 0.3s ease-in 0s 1 normal none running fadeInImage; width: 9.517ex; height: 2.509ex;'>가 존재한다.