IMO 1992 LL FIN14

Integers a1, a2, . . . , an satisfy |ak| = 1 and

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IMO 1992 LL FIN14

Origin: FIN

Problem

Integers a1, a2, . . . , an satisfy |ak| = 1 and n  k=1 akak+1ak+2ak+3 = 2, where an+j = aj. Prove that n ̸= 1992.