A046421 - OEIS

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A046421

Index of smallest repunit having exactly n prime factors (counted with multiplicity).

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

0,2

COMMENTS

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

LINKS

Table of n, a(n) for n=0..40.

Patrick De Geest, Repunits prime factors

Eric Weisstein's World of Mathematics, Repunit

EXAMPLE

For n = 5: R_6 = 111111 = 3*7*11*13*37 is the smallest repunit with five prime factors, so a(5) = 6.

PROG

(PARI) a(n) = my(k=1); while(bigomega((10^k - 1)/9) !=n, k++); k; \\ Michel Marcus, Apr 23 2017

CROSSREFS

Cf. A000042, A001222, A002275, A004022, A046053.

Initial terms of A004023, A046413, A046414, A046415, A046416, A046417, A046418, A046419.

Cf. A086565 (equivalent with distinct prime factors).

Sequence in context: A057776 A110362 A074478 * A132365 A129671 A117684

Adjacent sequences: A046418 A046419 A046420 * A046422 A046423 A046424

KEYWORD

nonn,more

AUTHOR

Patrick De Geest, Jul 15 1998

EXTENSIONS

a(1) = 2 inserted and a(19)-a(37) added by Ray Chandler, Apr 23 2017

a(38)-a(40) from Jinyuan Wang, Apr 17 2020

Name corrected by Felix Fröhlich, Jun 04 2022

STATUS

approved