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%I A114717 #42 Aug 06 2024 09:48:35
%S A114717 1,1,1,1,1,2,1,1,1,2,1,5,1,2,2,1,1,5,1,5,2,2,1,14,1,2,1,5,1,48,1,1,2,
%T A114717 2,2,42,1,2,2,14,1,48,1,5,5,2,1,42,1,5,2,5,1,14,2,14,2,2,1,2452,1,2,5,
%U A114717 1,2,48,1,5,2,48,1,462,1,2,5,5,2,48,1,42,1,2,1,2452,2,2,2,14,1,2452,2
%N A114717 Number of linear extensions of the divisor lattice of n.
%C A114717 Notice that only the powers of the primes determine a(n), so a(12) = a(75) = 5.
%C A114717 For prime powers, the lattice is a chain, so there is 1 linear extension.
%C A114717 a(p^1*q^n) = A000108(n+1), the Catalan numbers.
%C A114717 Alternatively, the number of ways to arrange the divisors of n in such a way that no divisor has any of its own divisors following it. E.g., for 12, the following five arrangements are possible: 1,2,3,4,6,12; 1,2,3,6,4,12; 1,2,4,3,6,12; 1,3,2,4,6,12 and 1,3,2,6,4,12. But 1,2,6,4,3,12 is not possible because 3 divides 6 but follows it. Thus a(12)=5. - _Antti Karttunen_, Jan 11 2006
%C A114717 For n = p1^r1 * p2^r2, the lattice is a grid (r1+1)*(r2+1), whose linear extensions are counted by ((r1+1)*(r2+1))!/Product_{k=0..r2} (r1+1+k)!/k!. Cf. A060854.
%D A114717 R. Stanley, Enumerative Combinatorics, Vol. 2, Proposition 7.10.3 and Vol. 1, Sec 3.5 Chains in Distributive Lattices.
%H A114717 Alois P. Heinz, <a href="/A114717/b114717.txt">Table of n, a(n) for n = 1..10000</a>
%H A114717 Graham Brightwell and Peter Winkler, <a href="https://doi.org/10.1007/BF00383444">Counting linear extensions</a>, Order 8 (1991), no. 3, 225-242.
%H A114717 Gary Pruesse and Frank Ruskey, <a href="https://citeseerx.ist.psu.edu/pdf/5e4ca7226502b6627dba415e1df2092888ed206d">Generating linear extensions fast</a>, SIAM J. Comput. 23 (1994), no. 2, 373-386.
%H A114717 <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>
%p A114717 with(numtheory):
%p A114717 b:= proc(s) option remember;
%p A114717       `if`(nops(s)<2, 1, add(`if`(nops(select(y->
%p A114717        irem(y, x)=0, s))=1, b(s minus {x}), 0), x=s))
%p A114717     end:
%p A114717 a:= proc(n) local l, m;
%p A114717       l:= sort(ifactors(n)[2], (x, y)-> x[2]>y[2]);
%p A114717       m:= mul(ithprime(i)^l[i][2], i=1..nops(l));
%p A114717       b(divisors(m) minus {1, m})
%p A114717     end:
%p A114717 seq(a(n), n=1..100);  # _Alois P. Heinz_, Jun 29 2012
%t A114717 b[s_List] := b[s] = If[Length[s]<2, 1, Sum[If[Length[Select[s, Mod[#, x] == 0 &]] == 1, b[Complement[s, {x}]], 0], {x, s}]]; a[n_] := Module[{l, m}, l = Sort[ FactorInteger[n], #1[[2]] > #2[[2]] &]; m = Product[Prime[i]^l[[i]][[2]], {i, 1, Length[l]}]; b[Divisors[m] // Rest // Most]]; Table[a[n], {n, 1, 100}] (* _Jean-François Alcover_, May 28 2015, after _Alois P. Heinz_ *)
%Y A114717 Cf. A060854, A114714, A114715, A114716, A119842.
%K A114717 nonn
%O A114717 1,6
%A A114717 _Mitch Harris_ and _Antti Karttunen_, Dec 27 2005

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