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A075023
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a(1) = 1, a(n) = the smallest prime divisor of the number C(n) = concatenation of 1,2,3... up to n then reverse up to 1.
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1
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1, 11, 3, 11, 41, 3, 239, 11, 3, 12345678910987654321, 7, 3, 1109, 7, 3, 71, 7, 3, 251, 7, 3, 70607, 7, 3
(list; graph; refs; listen; history; internal format)
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