• 42 can be written using four 4's:

See also 113.

• Deleting all the even digits from 2⁴² = 4398046511104 we obtain a prime (395111).

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42 has 8 divisors (see below), whose sum is σ = 96. Its totient is φ = 12.

The previous prime is 41. The next prime is 43. The reversal of 42 is 24.

Added to its reverse (24) it gives a triangular number (66 = T₁₁).

42 is nontrivially palindromic in base 4 and base 13.

42 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

42 is an esthetic number in base 2, because in such base its adjacent digits differ by 1.

It is an interprime number because it is at equal distance from previous prime (41) and next prime (43).

It is a sphenic number, since it is the product of 3 distinct primes.

42 is an admirable number.

It is the 5-th Catalan number.

It is a Harshad number since it is a multiple of its sum of digits (6), and also a Moran number because the ratio is a prime number: 7 = 42 / (4 + 2).

42 is an idoneal number.

It is a cake number, because a cake can be divided into 42 parts by 6 planar cuts.

42 is an undulating number in base 2.

It is a partition number, being equal to the number of ways a set of 10 identical objects can be partitioned into subset.

42 is a nontrivial repdigit in base 4 and base 13.

It is a plaindrome in base 4, base 9, base 11, base 12, base 13, base 15 and base 16.

It is a nialpdrome in base 4, base 6, base 7, base 8, base 10, base 13 and base 14.

It is a zygodrome in base 4 and base 13.

It is a self number, because there is not a number n which added to its sum of digits gives 42.

It is a pernicious number, because its binary representation contains a prime number (3) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3 + ... + 9.

It is an arithmetic number, because the mean of its divisors is an integer number (12).

It is a pronic number, being equal to 6×7.

It is a practical number, because each smaller number is the sum of distinct divisors of 42, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (48).

42 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

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

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

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

The sum of its prime factors is 12.

The product of its digits is 8, while the sum is 6.

The square root of 42 is about 6.4807406984. The cubic root of 42 is about 3.4760266449.

Adding to 42 its reverse (24), we get a palindrome (66).

The spelling of 42 in words is "forty-two", and thus it is an aban number, an eban number, an iban number, and an uban number.