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The seventh number is 6136. Each digit is its own separate sequence, as follows: 6, 4, 6, 4, 6, 4... is just a sequence alternating 6 and 4. 2, 2, 3, 1, 3, 1... the sum of the four digits increases by 1 modulo 9 every time. 1, 1, 2, 3, 5, 8... is the Fibonacci numbers. Since the next one is 13...