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A330990 Numbers whose inverse prime shadow (A181821) has its number of factorizations into factors > 1 (A001055) equal to a power of 2 (A000079). 7
1, 2, 3, 4, 6, 15, 44 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

A prime index of n is a number m such that prime(m) divides n. The multiset of prime indices of n is row n of A112798. The inverse prime shadow of n is the least number whose prime exponents are the prime indices of n.

LINKS

Table of n, a(n) for n=1..7.

FORMULA

A001055(A181821(a(n))) = 2^k for some k >= 0.

EXAMPLE

The factorizations of A181821(n) for n = 1, 2, 3, 4, 6, 15:

  ()  (2)  (4)    (6)    (12)     (72)

           (2*2)  (2*3)  (2*6)    (8*9)

                         (3*4)    (2*36)

                         (2*2*3)  (3*24)

                                  (4*18)

                                  (6*12)

                                  (2*4*9)

                                  (2*6*6)

                                  (3*3*8)

                                  (3*4*6)

                                  (2*2*18)

                                  (2*3*12)

                                  (2*2*2*9)

                                  (2*2*3*6)

                                  (2*3*3*4)

                                  (2*2*2*3*3)

MATHEMATICA

facs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[facs[n/d], Min@@#>=d&]], {d, Rest[Divisors[n]]}]];

nrmptn[n_]:=Join@@MapIndexed[Table[#2[[1]], {#1}]&, If[n==1, {}, Flatten[Cases[FactorInteger[n]//Reverse, {p_, k_}:>Table[PrimePi[p], {k}]]]]];

Select[Range[100], IntegerQ[Log[2, Length[facs[Times@@Prime/@nrmptn[#]]]]]&]

CROSSREFS

The same for prime numbers (instead of powers of 2) is A330993,

Factorizations are A001055, with image A045782.

Numbers whose number of factorizations is a power of 2 are A330977.

The least number with exactly 2^n factorizations is A330989.

Cf. A033833, A045778, A045783, A181821, A305936, A318283, A318284, A330972, A330973, A330976, A330998, A331022.

Sequence in context: A056712 A204540 A280592 * A337129 A002087 A049288

Adjacent sequences:  A330987 A330988 A330989 * A330991 A330992 A330993

KEYWORD

nonn,more

AUTHOR

Gus Wiseman, Jan 07 2020

STATUS

approved

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Last modified December 6 22:42 EST 2021. Contains 349567 sequences. (Running on oeis4.)