21645261981 has 48 divisors (see below), whose sum is σ = 37557921024. Its totient is φ = 11744485632.

The previous prime is 21645261931. The next prime is 21645261991. The reversal of 21645261981 is 18916254612.

21645261981 is a `hidden beast` number, since 2 + 1 + 6 + 452 + 6 + 198 + 1 = 666.

It is not a de Polignac number, because 21645261981 - 2¹⁷ = 21645130909 is a prime.

It is a super-2 number, since 2×21645261981² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21645261931) by changing a digit.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 143995 + ... + 253031.

It is an arithmetic number, because the mean of its divisors is an integer number (782456688).

Almost surely, 2^21645261981 is an apocalyptic number.

21645261981 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21645261981 is a deficient number, since it is larger than the sum of its proper divisors (15912659043).

21645261981 is a wasteful number, since it uses less digits than its factorization.

21645261981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 109210 (or 109207 counting only the distinct ones).

The product of its digits is 207360, while the sum is 45.

The spelling of 21645261981 in words is "twenty-one billion, six hundred forty-five million, two hundred sixty-one thousand, nine hundred eighty-one".