A number theory problem by Swadesh Rath

find sum of all n < 17 for which 49 divides the function f(n) = n! + (n+1)! + (n+2)!


The answer is 62.

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1 solution

Tom Engelsman
Feb 7, 2017

The function f ( n ) f(n) can be expressed as;

f ( n ) = n ! + ( n + 1 ) ! + ( n + 2 ) ! = n ! + ( n + 1 ) n ! + ( n + 2 ) ( n + 1 ) n ! ; f(n) = n! + (n+1)! + (n+ 2)! = n! +(n+1)n! + (n+2)(n+1)n! ;

or [ 1 + ( n + 1 ) + ( n + 2 ) ( n + 1 ) ] n ! = ( n 2 + 4 n + 4 ) n ! = ( n + 2 ) 2 n ! [1 + (n+1) + (n+2)(n+1)]n! = (n^2 + 4n + 4)n! = (n+2)^2 \cdot n!

If we are interested in n 0 , 1 , 2 , 3 , . . . , 16 n \in {0, 1,2 ,3,..., 16} such that 49 f ( n ) 49 | f(n) , then we obtain:

f ( n ) 49 = ( n + 2 ) 2 n ! 7 2 = ( n + 2 7 ) 2 n ! \frac{f(n)}{49} = \frac{(n+2)^2 \cdot n!}{7^2} = (\frac{n+2}{7})^2 \cdot n!

which is an integer iff n = 5 , 12 , 14 , 15 , 16 n = 5, 12, 14, 15, 16 . The final sum is just 5 + 12 + 14 + 15 + 16 = 62 . 5 + 12 + 14 + 15 + 16 = \boxed{62}.

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