1638172909721 has 2 divisors, whose sum is σ = 1638172909722. Its totient is φ = 1638172909720.

The previous prime is 1638172909699. The next prime is 1638172909739. The reversal of 1638172909721 is 1279092718361.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 1067369994496 + 570802915225 = 1033136^2 + 755515^2 .

It is a cyclic number.

It is not a de Polignac number, because 1638172909721 - 2¹⁴ = 1638172893337 is a prime.

It is a super-2 number, since 2×1638172909721² (a number of 25 digits) contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (1638172909781) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 819086454860 + 819086454861.

It is an arithmetic number, because the mean of its divisors is an integer number (819086454861).

Almost surely, 2^{1638172909721} is an apocalyptic number.

It is an amenable number.

1638172909721 is a deficient number, since it is larger than the sum of its proper divisors (1).

1638172909721 is an equidigital number, since it uses as much as digits as its factorization.

1638172909721 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 2286144, while the sum is 56.

The spelling of 1638172909721 in words is "one trillion, six hundred thirty-eight billion, one hundred seventy-two million, nine hundred nine thousand, seven hundred twenty-one".