21431781 has 24 divisors (see below), whose sum is σ = 37462464. Its totient is φ = 11525760.

The previous prime is 21431777. The next prime is 21431821. The reversal of 21431781 is 18713412.

It is not a de Polignac number, because 21431781 - 2² = 21431777 is a prime.

It is a super-2 number, since 2×21431781² = 918642473663922, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21431701) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 8935 + ... + 11076.

It is an arithmetic number, because the mean of its divisors is an integer number (1560936).

Almost surely, 2^21431781 is an apocalyptic number.

21431781 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21431781 is a deficient number, since it is larger than the sum of its proper divisors (16030683).

21431781 is a wasteful number, since it uses less digits than its factorization.

21431781 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 20041 (or 20038 counting only the distinct ones).

The product of its digits is 1344, while the sum is 27.

The square root of 21431781 is about 4629.4471592189. The cubic root of 21431781 is about 277.7704789134.

The spelling of 21431781 in words is "twenty-one million, four hundred thirty-one thousand, seven hundred eighty-one".