cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

A306585 Start with n and find the LCM of n and A140635(n), and continue until a number m is reached such that A140635(m) = m.

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%I A306585 #42 Apr 09 2023 02:44:51
%S A306585 1,2,6,4,120,6,840,24,36,120,83160,12,1081080,840,120,16,294053760,36,
%T A306585 5587021440,60,840,83160,128501493120,24,900,1081080,7560,2520,
%U A306585 93163582512000,120,2888071057872000,10080,83160,294053760,840,36,106858629141264000,5587021440,1081080,120,4381203794791824000
%N A306585 Start with n and find the LCM of n and A140635(n), and continue until a number m is reached such that A140635(m) = m.
%C A306585 a(n) = n is a subsequence of minimal numbers A007416, i.e., when A140635(n) = n. It appears that p_n > log_2(sigma_0(a(p_n))) for all primes p_n, and that a(p_n) form an increasing subsequence satisfying a(k) < a(p_n) for all k < p_n. - _Hartmut F. W. Hoft_, Mar 14 2023
%H A306585 Hartmut F. W. Hoft, <a href="/A306585/b306585.txt">Table of n, a(n) for n = 1..135</a>
%e A306585 a(5) = 120 because:
%e A306585 A140635(5) = 2 and LCM of 5 and 2 is 10,
%e A306585 A140635(10) = 6 and LCM of 10 and 6 is 30,
%e A306585 A140635(30) = 24 and LCM of 30 and 24 is 120,
%e A306585 A140635(120) = 120 so a(5) = 120.
%e A306585 From _Hartmut F. W. Hoft_, Mar 14 2023: (Start)
%e A306585 Table of iteration steps starting at n and ending with fixed point a(n):
%e A306585     1  ...
%e A306585     2  ...
%e A306585     3    6   ...
%e A306585     4  ...
%e A306585     5   10    30   120    ...
%e A306585     6    6   ...
%e A306585     7   14    42   168    840     ...
%e A306585     8   24   ...
%e A306585     9   36   ...
%e A306585    10   30   120   ...
%e A306585    11   22    66   264   1320    9240   83160       ...
%e A306585    12  ...
%e A306585    13   26    78   312   1560   10920   98280   1081080  ...
%e A306585    14   42   168   840    ...
%e A306585    15   30   120   ...
%e A306585    16  ...
%e A306585   ... (End)
%t A306585 (* a005179[ ] based on the function by _Vaclav Kotesovec_ in A005179 *)
%t A306585 mp[1, m_] := {{}}; mp[n_, 1] := {{}}; mp[n_?PrimeQ, m_] := If[m<n, {}, {{n}}];
%t A306585 mp[n_, m_] := Join@@Table[Map[Prepend[#, d]&, mp[n/d, d]], {d, Select[Rest[Divisors[n]], #<=m&]}]; mp[n_] := mp[n, n];
%t A306585 a005179[n_] := Module[{mulpar}, mulpar=mp[n]-1; Min[Table[Product[Prime[s]^mulpar[[j, s]], {s, 1, Length[mulpar[[j]]]}], {j, 1, Length[mulpar]}]]]
%t A306585 a306585[n_] := FixedPoint[LCM[#, a005179[DivisorSigma[0, #]]]&, n]
%t A306585 Map[a306585, Range[37]]  (* _Hartmut F. W. Hoft_, Mar 14 2023 *)
%o A306585 (PARI)
%o A306585 s(n) = my(nd=numdiv(n)); for(k=1, n, if(numdiv(k) == nd, return(k))); \\ A140635
%o A306585 a(n) = my(m=n, t=s(n)); while(1, m=lcm(m, t); t=s(m); if(m==t, return(m))); \\ _Daniel Suteu_, Feb 25 2019
%Y A306585 Cf. A005179, A007416, A140635.
%K A306585 nonn
%O A306585 1,2
%A A306585 _J. Lowell_, Feb 25 2019
%E A306585 a(13)-a(37) from _Rémy Sigrist_, Feb 25 2019
%E A306585 Terms a(38) and beyond from _Hartmut F. W. Hoft_, Mar 14 2023