Subfactorial

Number Theory Level pending

Find a six-digit number represented as a b c e d f \overline{abcedf} that has the following property:

a b c d e f = ! a + ! b + ! c + ! d + ! e + ! f \overline{abcdef} = !a + !b + !c + !d + !e + !f

The exclamation mark in front of the number indicates a sub-factorial which is defined as follows:

! n = n ! ( 1 1 1 ! + 1 2 ! 1 3 ! + + ( 1 ) n 1 n ! ) !n = n! \left(1 - \frac 1{1!} + \frac 1{2!} - \frac 1{3!} + \cdots + (-1)^n \frac 1{n!} \right)

For examples: ! 2 = 1 !2=1 and ! 6 = 265 !6=265

The exclamation mark after the number denotes the factorial function ; for example 8 ! = 1 × 2 × 3 × × 8 8! = 1 \times 2 \times 3 \times \cdots \times 8 .


The answer is 148349.

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1 solution

Giorgio Coniglio
Aug 7, 2016

148349 = !1 + !4 + !8 + !3 + !4 + !9

How do you get the answer?

Anandmay Patel - 4 years, 10 months ago

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If you examine the sub-factorials of 1 to 9 you will see that !9 = 133 496 and !8 = 14 833 and if you multiply both by 6 you get respectively 800 976 and 88 998. Here you can see that 9 must be one of the digits, and most likely 8 as well, and you start working from there.

This number that can be written as a function where you have all his digits participating, is called a narcissist number . Another example is 48 625 = 4 5 4^5 + 8 2 8^2 + 6 6 6^6 + 2 8 2^8 + 5 4 5^4 . See, only the digits of a narcissist number are there.

Giorgio Coniglio - 4 years, 10 months ago

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