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.

A330936 Number of nontrivial factorizations of n into factors > 1.

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%I A330936 #4 Jan 05 2020 08:11:24
%S A330936 0,0,0,0,0,0,0,1,0,0,0,2,0,0,0,3,0,2,0,2,0,0,0,5,0,0,1,2,0,3,0,5,0,0,
%T A330936 0,7,0,0,0,5,0,3,0,2,2,0,0,10,0,2,0,2,0,5,0,5,0,0,0,9,0,0,2,9,0,3,0,2,
%U A330936 0,3,0,14,0,0,2,2,0,3,0,10,3,0,0,9,0,0
%N A330936 Number of nontrivial factorizations of n into factors > 1.
%C A330936 The trivial factorizations of a number are (1) the case with only one factor, and (2) the factorization into prime numbers.
%F A330936 For prime n, a(n) = 0; for nonprime n, a(n) = A001055(n) - 2.
%e A330936 The a(n) nontrivial factorizations of n = 8, 12, 16, 24, 36, 48, 60, 72:
%e A330936   (2*4)  (2*6)  (2*8)    (3*8)    (4*9)    (6*8)      (2*30)    (8*9)
%e A330936          (3*4)  (4*4)    (4*6)    (6*6)    (2*24)     (3*20)    (2*36)
%e A330936                 (2*2*4)  (2*12)   (2*18)   (3*16)     (4*15)    (3*24)
%e A330936                          (2*2*6)  (3*12)   (4*12)     (5*12)    (4*18)
%e A330936                          (2*3*4)  (2*2*9)  (2*3*8)    (6*10)    (6*12)
%e A330936                                   (2*3*6)  (2*4*6)    (2*5*6)   (2*4*9)
%e A330936                                   (3*3*4)  (3*4*4)    (3*4*5)   (2*6*6)
%e A330936                                            (2*2*12)   (2*2*15)  (3*3*8)
%e A330936                                            (2*2*2*6)  (2*3*10)  (3*4*6)
%e A330936                                            (2*2*3*4)            (2*2*18)
%e A330936                                                                 (2*3*12)
%e A330936                                                                 (2*2*2*9)
%e A330936                                                                 (2*2*3*6)
%e A330936                                                                 (2*3*3*4)
%t A330936 facs[n_]:=If[n<=1,{{}},Join@@Table[Map[Prepend[#,d]&,Select[facs[n/d],Min@@#>=d&]],{d,Rest[Divisors[n]]}]];
%t A330936 Table[Length[DeleteCases[Rest[facs[n]],{_}]],{n,100}]
%Y A330936 Positions of nonzero terms are A033942.
%Y A330936 Positions of 1's are A030078.
%Y A330936 Positions of 2's are A054753.
%Y A330936 Nontrivial integer partitions are A007042.
%Y A330936 Nontrivial set partitions are A008827.
%Y A330936 Nontrivial divisors are A070824.
%Y A330936 Cf. A001055, A003238, A005121, A317145, A317176, A318812, A330665, A330935.
%K A330936 nonn
%O A330936 1,12
%A A330936 _Gus Wiseman_, Jan 04 2020