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A162306 Irregular triangle in which row n contains the numbers <= n whose prime factors are a subset of prime factors of n. 22
1, 1, 2, 1, 3, 1, 2, 4, 1, 5, 1, 2, 3, 4, 6, 1, 7, 1, 2, 4, 8, 1, 3, 9, 1, 2, 4, 5, 8, 10, 1, 11, 1, 2, 3, 4, 6, 8, 9, 12, 1, 13, 1, 2, 4, 7, 8, 14, 1, 3, 5, 9, 15, 1, 2, 4, 8, 16, 1, 17, 1, 2, 3, 4, 6, 8, 9, 12, 16, 18, 1, 19, 1, 2, 4, 5, 8, 10, 16, 20, 1, 3, 7, 9, 21, 1, 2, 4, 8, 11, 16, 22, 1, 23 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Row n begins with 1, ends with n, and has A010846(n) terms.

From Michael De Vlieger, Jul 08 2014: (Start)

Prime p has {1, p} and A010846(p) = 2.

Prime power p^e has {1, p, ..., p^e} and A010846(p^e) = A000005(p^e) = e + 1.

Composite c that are not prime powers have A010846(c) = A000005(c) + A243822(c), where A243822(c) is nonzero positive, since the minimum prime divisor p of c produces at least one semidivisor (e.g., p^2 < c). Thus these have the set of divisors of c and at least one semidivisor p^2. For squareful c that are not prime powers, p^2 may divide c, but p^3 does not. The minimum squareful c = 12, 2^3 does not divide 12 yet is less than 12 and is a product of the minimum prime divisor of 12. All other even squareful c admit a power of 2 that does not divide c, since there must be another prime divisor q > 2. (end)

Numbers 1 <= k <= n such that (floor(n^k/k) - floor((n^k - 1)/k)) = 1. - Michael De Vlieger, May 26 2016

Numbers 1 <= k <= n such that k | n^e with e >= 0. - Michael De Vlieger, May 29 2018

LINKS

T. D. Noe and Michael De Vlieger, Rows n = 1..1000 of triangle, flattened  (first rows n=1..200 from T. D. Noe)

FORMULA

Union of A027750 and nonzero terms of A272618.

EXAMPLE

n =  6, a(n) = {1, 2, 3, 4, 6}.

n =  7, a(n) = {1, 7}.

n =  8, a(n) = {1, 2, 4, 8}.

n =  9, a(n) = {1, 3, 9}.

n = 10, a(n) = {1, 2, 4, 5, 8, 10}.

n = 11, a(n) = {1, 11}.

n = 12, a(n) = {1, 2, 3, 4, 6, 8, 9, 12}.

MAPLE

A:= proc(n) local F, S, s, j, p;

  F:= numtheory:-factorset(n);

  S:= {1};

  for p in F do

    S:= {seq(seq(s*p^j, j=0..floor(log[p](n/s))), s=S)}

  od;

  S

end proc; map(op, [seq(A(n), n=1..100)]); # Robert Israel, Jul 15 2014

MATHEMATICA

pf[n_] := If[n==1, {}, Transpose[FactorInteger[n]][[1]]]; SubsetQ[lst1_, lst2_] := Intersection[lst1, lst2]==lst1; Flatten[Table[pfn=pf[n]; Select[Range[n], SubsetQ[pf[ # ], pfn] &], {n, 27}]]

(* Second program: *)

Table[Select[Range@ n, PowerMod[n, Floor@ Log2@ n, #] == 0 &], {n,

   23}] // Flatten (* Michael De Vlieger, May 29 2018 *)

CROSSREFS

Cf. A010846 (number of terms in row n), A027750 (terms k that divide n), A243103 (product of terms in row n), A244974 (sum of terms in row n), A272618 (terms k that do not divide n).

Sequence in context: A057059 A169896 A210208 * A233773 A027750 A275055

Adjacent sequences:  A162303 A162304 A162305 * A162307 A162308 A162309

KEYWORD

nonn,tabf,look

AUTHOR

T. D. Noe, Jun 30 2009

STATUS

approved

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Last modified October 23 18:36 EDT 2018. Contains 316529 sequences. (Running on oeis4.)