L'arrel n-èsima d'un nombre es defineix així:

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mroot mathcolor=¨#003300¨»«mi mathvariant=¨bold¨»x«/mi»«mi mathvariant=¨bold¨»n«/mi»«/mroot»«mo mathvariant=¨bold¨ mathcolor=¨#003300¨»=«/mo»«mi mathvariant=¨bold¨ mathcolor=¨#003300¨»y«/mi»«mo mathvariant=¨bold¨ mathcolor=¨#003300¨»§#160;«/mo»«mo mathvariant=¨bold¨ mathcolor=¨#003300¨»§#8660;«/mo»«mi mathvariant=¨bold¨ mathcolor=¨#003300¨»x«/mi»«mo mathvariant=¨bold¨ mathcolor=¨#003300¨»§#160;«/mo»«mo mathvariant=¨bold¨ mathcolor=¨#003300¨»=«/mo»«mo mathvariant=¨bold¨ mathcolor=¨#003300¨»§#160;«/mo»«msup mathcolor=¨#003300¨»«mi mathvariant=¨bold¨ mathcolor=¨#003300¨»y«/mi»«mi mathvariant=¨bold¨»n«/mi»«/msup»«/math»

Qualsevol arrel es pot escriure com un exponent fraccionari

i viceversa:

«math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mroot mathcolor=¨#003300¨»«msup»«mn mathvariant=¨bold¨»3«/mn»«mn mathvariant=¨bold¨»5«/mn»«/msup»«mn mathvariant=¨bold¨»7«/mn»«/mroot»«mo mathvariant=¨bold¨ mathcolor=¨#003300¨»=«/mo»«msup mathcolor=¨#003300¨»«mn mathvariant=¨bold¨ mathcolor=¨#003300¨»3«/mn»«mfrac bevelled=¨true¨»«mn mathvariant=¨bold¨»5«/mn»«mn mathvariant=¨bold¨»7«/mn»«/mfrac»«/msup»«/math»