EN
Let p > 3 be a prime, and let Rₚ be the set of rational numbers whose denominator is not divisible by p. Let {Pₙ(x)} be the Legendre polynomials. In this paper we mainly show that for m,n,t ∈ Rₚ with m≢ 0 (mod p),
$P_{[p/6]}(t) ≡ -(3/p)∑_{x=0}^{p-1} ((x³-3x+2t)/p) (mod p)$
and
$(∑_{x=0}^{p-1} ((x³+mx+n)/p))² ≡ ((-3m)/p) ∑_{k=0}^{[p/6]} \binom{2k}{k}\binom{3k}{k}\binom{6k}{3k} ((4m³+27n²)/(12³·4m³))^k (mod p)$,
where (a/p) is the Legendre symbol and [x] is the greatest integer function. As an application we solve some conjectures of Z. W. Sun and the author concerning $∑_{k=0}^{p-1}\binom{2k}{k}\binom{3k}{k}\binom{6k}{3k}/m^{k} (mod p²)$, where m is an integer not divisible by p.