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A326157 Squarefree numbers whose product of prime indices is twice their sum of prime indices. 2
65, 154, 190 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

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.

This sequence is finite. Proof: k = p_1*p_2*...*p_t is a term iff q_1*q_2*...*q_t = 2*(q_1 + q_2 + ... + q_t), where q_i = pi(p_i) and q_1 < q_2 < ... < q_t. If t = 2, then 1/2 = 1/q_1 + 1/q_2. Thus q_1 <= 3, we have k = prime(3)*prime(6) = 65. If t = 3, then 1/2 = 1/(q_1*q_2) + 1/(q_1*q_3) + 1/(q_2*q_3). Thus q_1*q_2 <= 5, we have k = prime(1)*prime(4)*prime(5) = 154 or k = prime(1)*prime(3)*prime(8) = 190. If t > 3, then 1/2 = Sum_{i=1..t} q_i/(q_1*q_2*...*q_t) < Sum_{i=1..t} i/t! < 1/2, a contradiction. - Jinyuan Wang, Jun 27 2020

LINKS

Table of n, a(n) for n=1..3.

FORMULA

A003963(a(n)) = 2 * A056239(a(n)).

EXAMPLE

The sequence of terms together with their prime indices starts:

   65: {3,6}

  154: {1,4,5}

  190: {1,3,8}

MAPLE

q:= n-> (l-> andmap(i-> i[2]=1, l) and (h-> mul(i, i=h)=2*add(i,

        i=h))(map(i-> numtheory[pi](i[1]), l)))(ifactors(n)[2]):

select(q, [$1..1000])[];  # Alois P. Heinz, Sep 12 2019

MATHEMATICA

primeMS[n_]:=If[n==1, {}, Flatten[Cases[FactorInteger[n], {p_, k_}:>Table[PrimePi[p], {k}]]]];

Select[Range[10000], SquareFreeQ[#]&&SameQ[Times@@primeMS[#], 2*Plus@@primeMS[#]]&]

CROSSREFS

Intersection of A005117 and A326151.

Product of prime indices is A003963.

Sum of prime indices is A056239.

Cf. A000720, A001222, A069016, A112798, A301987, A325041, A325042, A326152.

Sequence in context: A121944 A044316 A044697 * A044397 A044778 A054902

Adjacent sequences:  A326154 A326155 A326156 * A326158 A326159 A326160

KEYWORD

nonn,bref,fini,full

AUTHOR

Gus Wiseman, Sep 12 2019

STATUS

approved

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Last modified August 12 06:22 EDT 2020. Contains 336438 sequences. (Running on oeis4.)