81720421 has 8 divisors (see below), whose sum is σ = 83856024. Its totient is φ = 79591680.

The previous prime is 81720409. The next prime is 81720427. The reversal of 81720421 is 12402718.

It is a happy number.

It can be written as a sum of positive squares in 4 ways, for example, as 16900 + 81703521 = 130^2 + 9039^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 81720421 - 2⁷ = 81720293 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81720427) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 29721 + ... + 32353.

It is an arithmetic number, because the mean of its divisors is an integer number (10482003).

Almost surely, 2^81720421 is an apocalyptic number.

It is an amenable number.

81720421 is a deficient number, since it is larger than the sum of its proper divisors (2135603).

81720421 is a wasteful number, since it uses less digits than its factorization.

81720421 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3431.

The product of its (nonzero) digits is 896, while the sum is 25.

The square root of 81720421 is about 9039.9347895878. The cubic root of 81720421 is about 433.9538360434.

The spelling of 81720421 in words is "eighty-one million, seven hundred twenty thousand, four hundred twenty-one".