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A046421
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Index of smallest repunit having exactly n prime factors (counted with multiplicity).
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0
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1, 2, 3, 13, 8, 6, 15, 12, 28, 18, 24, 32, 36, 30, 54, 42, 78, 100, 72, 176, 60, 208, 84, 132, 160, 198, 120, 204, 216, 308, 168, 280, 306, 180, 210, 264, 270, 252, 378, 336, 300
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OFFSET
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0,2
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COMMENTS
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a(40) = 300; all other subsequent terms are > 322. - Ray Chandler, Apr 23 2017
a(41) <= 684, a(42) <= 546, a(43) <= 528, a(44) <= 462, a(45) = 360, a(46) <= 576, a(47) <= 624, a(48) <= 768. - Daniel Suteu, Jan 21 2023
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LINKS
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Eric Weisstein's World of Mathematics, Repunit
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EXAMPLE
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For n = 5: R_6 = 111111 = 3*7*11*13*37 is the smallest repunit with five prime factors, so a(5) = 6.
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PROG
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(PARI) a(n) = my(k=1); while(bigomega((10^k - 1)/9) !=n, k++); k; \\ Michel Marcus, Apr 23 2017
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CROSSREFS
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Cf. A086565 (equivalent with distinct prime factors).
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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a(1) = 2 inserted and a(19)-a(37) added by Ray Chandler, Apr 23 2017
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STATUS
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approved
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