81121721 has 2 divisors, whose sum is σ = 81121722. Its totient is φ = 81121720.

The previous prime is 81121687. The next prime is 81121751. The reversal of 81121721 is 12712118.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 74304400 + 6817321 = 8620^2 + 2611^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-81121721 is a prime.

It is not a weakly prime, because it can be changed into another prime (81121751) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40560860 + 40560861.

It is an arithmetic number, because the mean of its divisors is an integer number (40560861).

Almost surely, 2^81121721 is an apocalyptic number.

It is an amenable number.

81121721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81121721 is an equidigital number, since it uses as much as digits as its factorization.

81121721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 224, while the sum is 23.

The square root of 81121721 is about 9006.7597392181. The cubic root of 81121721 is about 432.8914935584.

Adding to 81121721 its reverse (12712118), we get a palindrome (93833839).

The spelling of 81121721 in words is "eighty-one million, one hundred twenty-one thousand, seven hundred twenty-one".