Binomial coefficient modulo a prime

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SUMMARY

The discussion confirms that the binomial coefficient bin(2p, p) is congruent to 2 modulo p for any prime p, including cases where p is greater than or equal to 3. This conclusion is derived from Fermat's Little Theorem, which states that x^p ≡ x (mod p) for a prime p. The example of 4 choose 2, which equals 6 and is congruent to 2 mod 2, further illustrates this property.

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erszega
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A question:

Let bin(a,b) denote the binomial coefficient a! / ( b! (a - b)! ).

Is it true that

bin( 2p, p ) = 2 (mod p) if p is prime and p>=3 ?
 
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Yes, it's fermat's little theorem: x^p=x mod p, for p a prime, hence

(1+x)^2p = (1+x^p)^2 = 1+2x^p+x^{2p} mod p

note your requirement on p>=3 is not necessary. 4 choose 2 =6 whcih is congruent to 2 mod 2 as well.
 

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