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 A249770 Irregular triangle read by rows: T(n,k) is the number of Abelian groups of order n with k invariant factors (2 <= n, 1 <= k). 4
 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 1, 1, 1, 1, 3, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,21 COMMENTS The length of n-th row is A051903(n) and its last element is A249773(A101296(n)). T(n,k) depends only on k and the prime signature of n. LINKS Álvar Ibeas, Rows n=2..58654, flattened FORMULA T(n,k) = A249771(A101296(n),k). T(n,1) = 1. If k > 1 and n = Product(p_i^e_i), T(n,k) = Sum(Product(A008284(e_i,k), i in I) * Product(A026820(e_i,k-1), i not in I)), where the sum is taken over nonempty subsets I of {1,...,omega(n)}. If p is prime and gcd(p,n) = 1, T(pn,k) = T(n,k). Dirichlet g.f. of column sums: zeta(s)zeta(2s)···zeta(ms) = 1 + Sum_{n >= 2} (Sum_{k=1..m} T(n,k)) / n^s. T(n,1) + T(n,2) = A046951(n) EXAMPLE First rows: 1; 1; 1,1; 1; 1; 1; 1,1,1; 1,1; 1; 1; 1,1; 1; 1; 1; 1,2,1,1; 1; ... MATHEMATICA f[{x_, y_}] := x^IntegerPartitions[y]; g[n_] := FactorInteger[n][[1, 1]]; h[list_] := Apply[Times, Map[PadRight[#, Max[Map[Length, SplitBy[list, g]]], 1] &, SplitBy[list, g]]]; t[list_] := Tally[Map[Length, list]][[All, 2]]; Map[t, Table[Map[h, Join @@@ Tuples[Map[f, FactorInteger[n]]]], {n, 2, 50}]] // Grid (* Geoffrey Critzer, Nov 26 2015 *) CROSSREFS Refinement of A000688. Cf. A008284, A051903, A026820, A101296, A249771, A249773, A046951, A264809. Sequence in context: A078315 A288120 A156264 * A298481 A324872 A307608 Adjacent sequences:  A249767 A249768 A249769 * A249771 A249772 A249773 KEYWORD nonn,tabf AUTHOR Álvar Ibeas, Nov 06 2014 STATUS approved

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Last modified April 21 10:51 EDT 2019. Contains 322328 sequences. (Running on oeis4.)