21601991 has 16 divisors (see below), whose sum is σ = 23302080. Its totient is φ = 19979520.

The previous prime is 21601973. The next prime is 21602003. The reversal of 21601991 is 19910612.

It is a cyclic number.

It is not a de Polignac number, because 21601991 - 2⁶ = 21601927 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 (21601691) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 124781 + ... + 124953.

It is an arithmetic number, because the mean of its divisors is an integer number (1456380).

Almost surely, 2^21601991 is an apocalyptic number.

21601991 is a deficient number, since it is larger than the sum of its proper divisors (1700089).

21601991 is a wasteful number, since it uses less digits than its factorization.

21601991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 346.

The product of its (nonzero) digits is 972, while the sum is 29.

The square root of 21601991 is about 4647.7942080088. The cubic root of 21601991 is about 278.5038866088.

The spelling of 21601991 in words is "twenty-one million, six hundred one thousand, nine hundred ninety-one".