1216257721 has 4 divisors (see below), whose sum is σ = 1390008832. Its totient is φ = 1042506612.

The previous prime is 1216257719. The next prime is 1216257769. The reversal of 1216257721 is 1277526121.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 1277526121 = 37 ⋅34527733.

It is not a de Polignac number, because 1216257721 - 2¹ = 1216257719 is a prime.

It is a super-2 number, since 2×1216257721² = 2958565687784227682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1216257781) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 86875545 + ... + 86875558.

It is an arithmetic number, because the mean of its divisors is an integer number (347502208).

Almost surely, 2^1216257721 is an apocalyptic number.

It is an amenable number.

1216257721 is a deficient number, since it is larger than the sum of its proper divisors (173751111).

1216257721 is an equidigital number, since it uses as much as digits as its factorization.

1216257721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 173751110.

The product of its digits is 11760, while the sum is 34.

The square root of 1216257721 is about 34874.8866808195. The cubic root of 1216257721 is about 1067.4360602084.

The spelling of 1216257721 in words is "one billion, two hundred sixteen million, two hundred fifty-seven thousand, seven hundred twenty-one".