Decades: 1170s 1180s 1190s 1200s 1210s - 1220s - 1230s 1240s 1250s 1260s 1270s
Years: 1221 1222 1223 1224 1225 - 1226 - 1227 1228 1229 1230 1231
Births
Deaths
... by (f1 + f2)(x) = f1(x) + f2(x)). If f : V → W is linear and a is an element of the ground field K, then the map af, defined by (af)(x) = a (f(x)), is also ...