Search: id:A005580
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%I A005580 M2740
%S A005580 3,8,21,54,141,372,995,2697,7397,20502,57347,161658
%N A005580 Least number of distinct prime factors in odd numbers having an abundancy 
               index > n.
%C A005580 The abundancy index of a number k is sigma(k)/k. - T. D. Noe (noe(AT)sspectra.com), 
               May 08 2006
%D A005580 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A005580 Laatsch, Richard; Measuring the abundancy of integers. Math. Mag. 59 
               (1986), no. 2, 84-92.
%F A005580 a(n)=A005579(2n)-1 - T. D. Noe (noe(AT)sspectra.com), May 08 2006
%t A005580 prod=1; k=1; Table[While[prod<=n, k++; prod=prod*Prime[k]/(Prime[k]-1)]; 
               k, {n,2,12}] - T. D. Noe (noe(AT)sspectra.com), May 08 2006
%Y A005580 Cf. A005579 (least number of distinct prime factors in even numbers having 
               an abundancy index > n).
%Y A005580 Sequence in context: A127358 A077849 A135473 this_sequence A027932 A084625 
               A001906
%Y A005580 Adjacent sequences: A005577 A005578 A005579 this_sequence A005581 A005582 
               A005583
%K A005580 nonn
%O A005580 2,1
%A A005580 N. J. A. Sloane (njas(AT)research.att.com).
%E A005580 Edited by T. D. Noe (noe(AT)sspectra.com), May 08 2006

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