http://z5lcip2ba3ec6v5za5snjzncpvr6ocyvqnkox2alerut6ruozpjtvkqd.onion/math/catalog
Rings are generalizations of the integers. Rings must satisfy (a+b)+c=a+(b+c), a+0=a, a+(-a)=0, a+b=b+a, (a*b)*c=a*(b*c), a*1=a, 1*a=a, a*(b+c)=a*b+a*c, (a+b)*c=a*c+b*c and in some definitions also a*b=b*a but don't need to have a division operation.