21951810 has 80 divisors (see below), whose sum is σ = 60557112. Its totient is φ = 5702400.

The previous prime is 21951803. The next prime is 21951817. The reversal of 21951810 is 1815912.

It is an interprime number because it is at equal distance from previous prime (21951803) and next prime (21951817).

It can be written as a sum of positive squares in 4 ways, for example, as 15186609 + 6765201 = 3897^2 + 2601^2 .

It is a super-2 number, since 2×21951810² = 963763924552200, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is not an unprimeable number, because it can be changed into a prime (21951817) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 39 ways as a sum of consecutive naturals, for example, 32880 + ... + 33540.

Almost surely, 2^21951810 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21951810, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30278556).

21951810 is an abundant number, since it is smaller than the sum of its proper divisors (38605302).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21951810 is a wasteful number, since it uses less digits than its factorization.

21951810 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 721 (or 712 counting only the distinct ones).

The product of its (nonzero) digits is 720, while the sum is 27.

The square root of 21951810 is about 4685.2758723473. The cubic root of 21951810 is about 279.9991921745. Note that the first 3 decimals are identical.

It can be divided in two parts, 2195 and 1810, that added together give a triangular number (4005 = T₈₉).

The spelling of 21951810 in words is "twenty-one million, nine hundred fifty-one thousand, eight hundred ten".