81145721 has 2 divisors, whose sum is σ = 81145722. Its totient is φ = 81145720.

The previous prime is 81145717. The next prime is 81145763. The reversal of 81145721 is 12754118.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 81090025 + 55696 = 9005^2 + 236^2 .

It is a cyclic number.

It is not a de Polignac number, because 81145721 - 2² = 81145717 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (81145781) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40572860 + 40572861.

It is an arithmetic number, because the mean of its divisors is an integer number (40572861).

Almost surely, 2^81145721 is an apocalyptic number.

It is an amenable number.

81145721 is a deficient number, since it is larger than the sum of its proper divisors (1).

81145721 is an equidigital number, since it uses as much as digits as its factorization.

81145721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2240, while the sum is 29.

The square root of 81145721 is about 9008.0919733315. The cubic root of 81145721 is about 432.9341799122.

Adding to 81145721 its reverse (12754118), we get a palindrome (93899839).

The spelling of 81145721 in words is "eighty-one million, one hundred forty-five thousand, seven hundred twenty-one".