81720409 has 2 divisors, whose sum is σ = 81720410. Its totient is φ = 81720408.

The previous prime is 81720313. The next prime is 81720427. The reversal of 81720409 is 90402718.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 80946009 + 774400 = 8997^2 + 880^2 .

It is a cyclic number.

It is not a de Polignac number, because 81720409 - 2¹⁷ = 81589337 is a prime.

It is not a weakly prime, because it can be changed into another prime (81720439) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 40860204 + 40860205.

It is an arithmetic number, because the mean of its divisors is an integer number (40860205).

Almost surely, 2^81720409 is an apocalyptic number.

It is an amenable number.

81720409 is a deficient number, since it is larger than the sum of its proper divisors (1).

81720409 is an equidigital number, since it uses as much as digits as its factorization.

81720409 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 4032, while the sum is 31.

The square root of 81720409 is about 9039.9341258662. The cubic root of 81720409 is about 433.9538148025.

The spelling of 81720409 in words is "eighty-one million, seven hundred twenty thousand, four hundred nine".