1221121721 has 4 divisors (see below), whose sum is σ = 1221440244. Its totient is φ = 1220803200.

The previous prime is 1221121691. The next prime is 1221121751. The reversal of 1221121721 is 1271211221.

It is a happy number.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1271211221 = 7 ⋅181601603.

It is an interprime number because it is at equal distance from previous prime (1221121691) and next prime (1221121751).

It can be written as a sum of positive squares in 2 ways, for example, as 429318400 + 791803321 = 20720^2 + 28139^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-1221121721 is a prime.

It is a super-2 number, since 2×1221121721² = 2982276514996003682, which contains 22 as substring.

It is a Duffinian number.

It is a junction number, because it is equal to n+sod(n) for n = 1221121693 and 1221121702.

It is not an unprimeable number, because it can be changed into a prime (1221121751) 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, 153440 + ... + 161201.

It is an arithmetic number, because the mean of its divisors is an integer number (305360061).

Almost surely, 2^1221121721 is an apocalyptic number.

It is an amenable number.

1221121721 is a deficient number, since it is larger than the sum of its proper divisors (318523).

1221121721 is an equidigital number, since it uses as much as digits as its factorization.

1221121721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 318522.

The product of its digits is 112, while the sum is 20.

The square root of 1221121721 is about 34944.5520932806. The cubic root of 1221121721 is about 1068.8571140503.

Adding to 1221121721 its reverse (1271211221), we get a palindrome (2492332942).

The spelling of 1221121721 in words is "one billion, two hundred twenty-one million, one hundred twenty-one thousand, seven hundred twenty-one".