This site is supported by donations to The OEIS Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A087134 Smallest number having exactly n divisors that are not greater than the number's greatest prime factor. 6
 1, 2, 6, 20, 42, 84, 156, 312, 684, 1020, 1380, 1860, 3480, 3720, 4920, 7320, 10980, 14640, 16920, 21960, 26280, 34920, 45720, 59640, 69840, 89880, 106680, 125160, 145320, 177240, 213360, 244440, 269640, 354480, 320040, 375480, 435960, 456120, 531720, 647640 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A087133(a(n))=n. Also smallest number such that the n-th divisor is prime. - Reinhard Zumkeller, May 15 2006 From David A. Corneth, Jan 22 2019: (Start) For the first 10000 terms except 1, a(n) is of the form A025487(k) * p where p is the smallest prime larger than the n-th divisor and, if the (n+1)-th divisor exists, less than that divisor. This sequence isn't a sequence of indices of records to A087133 as it's not monotonically increasing; 354480 = a(34) > a(35) = 320040. (End) LINKS David A. Corneth, Table of n, a(n) for n = 1..10000 Eric Weisstein's World of Mathematics, Divisor Function Eric Weisstein's World of Mathematics, Greatest Prime Factor EXAMPLE a(3) = A119313(1) = 6. MATHEMATICA With[{s = Array[Function[{d, p}, LengthWhile[d, # < p &]] @@ {#, SelectFirst[Reverse@ #, PrimeQ]} &@ Divisors@ # &, 10^6]}, Array[FirstPosition[s, #][] &, Max@ s + 1, 0]] (* Michael De Vlieger, Jan 23 2019 *) PROG (PARI) a087133(n) = if (n==1, 1, my(f = factor(n), gpf = f[#f~, 1]); sumdiv(n, d, d <= gpf)); a(n) = my(k = 1); while (a087133(k) != n, k++); k; \\ Michel Marcus, Sep 21 2014 CROSSREFS Cf. A006530, A025487, A087133, A119311, A119312. Sequence in context: A087150 A323724 A214307 * A036689 A226326 A139115 Adjacent sequences:  A087131 A087132 A087133 * A087135 A087136 A087137 KEYWORD nonn AUTHOR Reinhard Zumkeller, Aug 17 2003 EXTENSIONS More terms from Reinhard Zumkeller, May 15 2006 More terms from Michel Marcus, Sep 21 2014 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified March 20 13:18 EDT 2019. Contains 321345 sequences. (Running on oeis4.)