1991808405 has 18 divisors (see below), whose sum is σ = 3452986914. Its totient is φ = 1062138144.

The previous prime is 1991808307. The next prime is 1991808409. The reversal of 1991808405 is 5048081991.

It can be written as a sum of positive squares in 3 ways, for example, as 182412036 + 1809396369 = 13506^2 + 42537^2 .

It is not a de Polignac number, because 1991808405 - 2¹¹ = 1991806357 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (45).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991808409) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 17 ways as a sum of consecutive naturals, for example, 296059 + ... + 302711.

Almost surely, 2^1991808405 is an apocalyptic number.

1991808405 is a gapful number since it is divisible by the number (15) formed by its first and last digit.

It is an amenable number.

1991808405 is a deficient number, since it is larger than the sum of its proper divisors (1461178509).

1991808405 is an frugal number, since it uses more digits than its factorization.

1991808405 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 13317 (or 6661 counting only the distinct ones).

The product of its (nonzero) digits is 103680, while the sum is 45.

The square root of 1991808405 is about 44629.6807629183. The cubic root of 1991808405 is about 1258.1985689436.

Multiplying 1991808405 by its sum of digits (45), we get a square (89631378225 = 299385²).

1991808405 divided by its sum of digits (45) gives a square (44262409 = 6653²).

Multiplying 1991808405 by its product of nonzero digits (103680), we get a square (206510695430400 = 14370480²).

The spelling of 1991808405 in words is "one billion, nine hundred ninety-one million, eight hundred eight thousand, four hundred five".