Back عدد غير ملموس Arabic অস্পৃশ্য সংখ্যা Bengali/Bangla Nedotknutelné číslo Czech Unberührbare Zahl German Nùmer mia tucàbil EML Número intocable Spanish Nombre intouchable French מספר נגיע HE Érinthetetlen számok Hungarian Numero intoccabile Italian
In mathematics, an untouchable number is a positive integer that cannot be expressed as the sum of all the proper divisors of any positive integer. That is, these numbers are not in the image of the aliquot sum function. Their study goes back at least to Abu Mansur al-Baghdadi (circa 1000 AD), who observed that both 2 and 5 are untouchable.[1]