It is a happy number.
It is a strong prime.
It is a cyclic number.
It is a congruent number.
Almost surely, 2121220111221591 is an apocalyptic number.
121220111221591 is an equidigital number, since it uses as much as digits as its factorization.
121220111221591 is an odious number, because the sum of its binary digits is odd.
The spelling of 121220111221591 in words is "one hundred twenty-one trillion, two hundred twenty billion, one hundred eleven million, two hundred twenty-one thousand, five hundred ninety-one".