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A237350
a(n) = the smallest number k such that Sum_{d|k} 1/tau(d) >= n.
4
1, 6, 24, 60, 180, 420, 840, 2520, 4620, 9240, 13860, 27720, 55440, 55440, 110880, 166320, 180180, 360360, 360360, 720720, 720720, 1441440, 1801800, 2162160, 3063060, 4084080, 6126120, 6126120, 6126120, 12252240, 12252240, 18378360, 24504480, 24504480, 30630600, 36756720
OFFSET
1,2
COMMENTS
Are there numbers n > 1 such that Sum_{d|n} 1/tau(d) is an integer?
Values of function F = Sum_{d|n} 1/tau(d) for some numbers according to their prime signature: F{} = 1; F{1} = 3/2; F{2} = 11/6; F{1, 1} = 9/4; F{3} = 25/12; F{2, 1} = 11/4; F{4} = 137/60; F{3, 1} = 25/8, ...
All terms are of the form Product_{j=1..k} prime(j)^e(j) where e(j+1)<= e(j), and thus products of (not necessarily distinct) primorials. - Robert Israel, Dec 21 2015
From David A. Corneth, Nov 05 2019: (Start)
Instead of checking all divisors of A025487(n), one could use A318277 to see how often each prime signature occurs as a divisor.
Knowing the lcm of the terms below some m drastically improves the possibility of finding terms. In hindsight, knowing the lcm of the terms below 10^25 yields having to consider 1056 terms of A025487 instead of 222124. Is there some way to accurately predict the lcm to improve computation? (End)
LINKS
David A. Corneth, Table of n, a(n) for n = 1..3338 (first 131 terms from Robert Israel, terms <= 10^25)
EXAMPLE
For n = 2; a(2) = 6 because 6 is the smallest number with Sum_{d|6} 1/tau(d) = 1/1 + 1/2 + 1/2 + 1/4 = 9/4 >= 2.
MAPLE
N:= 10^9: # to get all entries <= N
Primorials:= NULL:
p:= 2: P:= p:
while P <= N do
Primorials:= Primorials, P;
p:= nextprime(p);
P:= P*p;
od:
Primorials:= [Primorials]:
S:= {1}:
for i from 1 to nops(Primorials) do
S:= {seq(seq(s*Primorials[i]^j,
j = 0 .. floor(log[Primorials[i]](N/s))), s=S)}
od:
A:= NULL:
S:= sort(convert(S, list)):
xmax:= 0:
for s in S do
x:= floor(add(1/numtheory:-tau(d), d=numtheory:-divisors(s)));
if x > xmax then
A:= A, s$(x-xmax);
xmax:= x
fi
od:
A; # Robert Israel, Dec 21 2015
MATHEMATICA
s[1] = 1; s[n_] := DivisorSum[n, 1/DivisorSigma[0, #] &]; n = 1; k = 1; seq = {}; Do[While[s[k] < n, k++]; AppendTo[seq, k]; n++, {j, 1, 20}]; seq (* Amiram Eldar, Jan 30 2019 *)
PROG
(Magma) a:=1; S:=[a]; for n in [2..14] do k:=0; flag:= true; while flag do k+:=1; if &+[1/NumberOfDivisors(d): d in Divisors(k)] gt n then Append(~S, k); a:=k; flag:=false; end if; end while; end for; S;
(PARI) a(n) = {my(k=1); while(sumdiv(k, d, 1/numdiv(d)) < n, k++); k; } \\ Michel Marcus, Dec 20 2015
CROSSREFS
Cf. A265393 (a(n) = the smallest number k such that floor(Sum_{d|k} 1/tau(d)) = n).
Sequence in context: A258351 A130669 A214308 * A265393 A292908 A293017
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Dec 13 2015
EXTENSIONS
a(24)-a(30) from Michel Marcus, Dec 20 2015
a(31)-a(35) from Robert Israel, Dec 21 2015
Missing a(31) = 12252240 inserted in data section by Georg Fischer, Nov 05 2019
STATUS
approved