19951721 has 4 divisors (see below), whose sum is σ = 20097492. Its totient is φ = 19805952.

The previous prime is 19951709. The next prime is 19951733. The reversal of 19951721 is 12715991.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 12715991 = 47 ⋅270553.

It is an interprime number because it is at equal distance from previous prime (19951709) and next prime (19951733).

It can be written as a sum of positive squares in 2 ways, for example, as 5784025 + 14167696 = 2405^2 + 3764^2 .

It is a cyclic number.

It is not a de Polignac number, because 19951721 - 2¹⁸ = 19689577 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (19951021) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 72680 + ... + 72953.

It is an arithmetic number, because the mean of its divisors is an integer number (5024373).

Almost surely, 2^19951721 is an apocalyptic number.

It is an amenable number.

19951721 is a deficient number, since it is larger than the sum of its proper divisors (145771).

19951721 is a wasteful number, since it uses less digits than its factorization.

19951721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 145770.

The product of its digits is 5670, while the sum is 35.

The square root of 19951721 is about 4466.7349372892. The cubic root of 19951721 is about 271.2231700618.

The spelling of 19951721 in words is "nineteen million, nine hundred fifty-one thousand, seven hundred twenty-one".