Let be an odd prime. The remainder when is divided by is?
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( 2 p − 1 ) ! + 2 ( p ! ) + p = ( ( 2 p − 1 ) ( 2 p − 2 ) . . . ( p + 2 ) ( p + 1 ) ( p − 1 ) ! + 2 ( p − 1 ) ! + 1 ) p where ( 2 p − 1 ) ( 2 p − 2 ) . . . ( p + 2 ) ( p + 1 ) = ( p + p − 1 ) ( p + p − 2 ) . . . ( p + 2 ) ( p + 1 ) = ( x + p − 1 ) ( x + p − 2 ) . . . ( x + 2 ) ( x + 1 ) where x = p .
Note that the constant of this polynomial is ( p − 1 ) ! while its coefficient of x is:
1 ( p − 1 ) ! + 2 ( p − 1 ) ! + . . . + p − 2 ( p − 1 ) ! + p − 1 ( p − 1 ) ! = 1 ( p − 1 ) ! + p − 1 ( p − 1 ) ! + 2 ( p − 1 ) ! + p − 2 ( p − 1 ) ! + . . . = 1 ( p − 1 ) p ( p − 1 ) ! + 2 ( p − 2 ) p ( p − 1 ) ! + . . . which is divisible by p . Note that the last term involves 2 p − 1 which is why p must be an odd prime so that ( p − 1 ) must be even.
Now upon substitution of x = p , the coefficient of x will contribute another factor of p other than the existing x = p . Considering that the rest of the polynomial is also divisible by p 2 with all terms of power at least x 2 , we see that the expression is divisible by p 2 except for the constant term ( p − 1 ) ! .
( 2 p − 1 ) ( 2 p − 2 ) . . . ( p + 2 ) ( p + 1 ) ≡ ( p − 1 ) ! (mod p 2 ) ( 2 p − 1 ) ( 2 p − 2 ) . . . ( p + 2 ) ( p + 1 ) ( p − 1 ) ! + 2 ( p − 1 ) ! + 1 ≡ ( p − 1 ) ! ( p − 1 ) ! + 2 ( p − 1 ) ! + 1 ≡ ( ( p − 1 ) ! + 1 ) 2 (mod p 2 )
Considering that ( p − 1 ) ! ≡ − 1 (mod p ), we have p ∣ ( ( p − 1 ) ! + 1 ) thus p 2 ∣ ( ( p − 1 ) ! + 1 ) 2 . This means that p 2 ∣ ( ( 2 p − 1 ) ( 2 p − 2 ) . . . ( p + 2 ) ( p + 1 ) ( p − 1 ) ! + 2 ( p − 1 ) ! + 1 ) . Then p 2 + 1 ∣ ( ( 2 p − 1 ) ( 2 p − 2 ) . . . ( p + 2 ) ( p + 1 ) ( p − 1 ) ! + 2 ( p − 1 ) ! + 1 ) p which means that p 3 ∣ ( ( 2 p − 1 ) ( 2 p − 2 ) . . . ( p + 2 ) ( p + 1 ) ( p − 1 ) ! + 2 ( p − 1 ) ! + 1 ) p . Indeed, p 3 ∣ ( ( 2 p − 1 ) ! + 2 ( p ! ) + p ) . Thus the remainder when ( 2 p − 1 ) ! + 2 ( p ! ) + p is divided by p 3 is 0 .