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find the number of squares

Source: Austria 1991

July 10, 2009
inductionmodular arithmeticquadraticsnumber theory unsolvednumber theory

Problem Statement

Find the number of squares in the sequence given by a_0\equal{}91 and a_{n\plus{}1}\equal{}10a_n\plus{}(\minus{}1)^n for n≥0. n \ge 0.
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