find sum of all n < 17 for which 49 divides the function f(n) = n! + (n+1)! + (n+2)!
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The function f ( n ) can be expressed as;
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 !
If we are interested in n ∈ 0 , 1 , 2 , 3 , . . . , 1 6 such that 4 9 ∣ f ( n ) , then we obtain:
4 9 f ( n ) = 7 2 ( n + 2 ) 2 ⋅ n ! = ( 7 n + 2 ) 2 ⋅ n !
which is an integer iff n = 5 , 1 2 , 1 4 , 1 5 , 1 6 . The final sum is just 5 + 1 2 + 1 4 + 1 5 + 1 6 = 6 2 .