840 (eight hundred [and] forty) is thenatural number following 839 and preceding 841.

• It is aneven number.
• It is apractical number.
• It is acongruent number.
• It is the 15thhighly composite number,^[1] with 32 divisors: 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 15, 20, 21, 24, 28, 30, 35, 40, 42, 56, 60, 70, 84, 105, 120, 140, 168, 210, 280, 420, 840. Since the sum of its divisors (excluding the number itself) 2040 > 840
• It is anabundant number and also asuperabundant number.^[2]
• It is anidoneal number.^[3]
• It is theleast common multiple of the numbers from 1 to 8.^[4]
• It is the smallest number divisible by every natural number from 1 to 10, except 9.
• It is the number under 1000 with the most divisors, at 32.
• It is the largest numberk such that all coprimequadratic residues modulok are squares. In this case, they are 1, 121, 169, 289, 361 and 529.^[5]
• It is anevil number.
• It is apalindrome number and arepdigit number repeated in the positional numbering system in base 29 (SS) and in that in base 34 (OO).
• It is the sum of a twin prime (419 + 421).
• It is the triple-digit number with the most divisors at 32.

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