OFFSET
1,1
COMMENTS
Prime powers p^e where p is a prime and e >= 2 (A246547) form a subsequence.
For numbers whose prime factors set is {p_1, p_2, ..., p_r}, there exists a minimal element u such that k is a term iff k >= u. This smallest element u satisfies p_1*p_2*...*p_r < u <= (p_1*p_2*...*p_r)^2. These minimal elements are in A339794.
Table with percentage of terms <= 10^k for k = 1, 2, ..., 8, 9 (first rows coming from b-file):
+-------+------------------------+----------------------------+
| k |number of terms <= 10^k |percentage of terms <= 10^k |
| | | % |
+-------+------------------------+----------------------------+
| 1 | 3 | 30 |
| 2 | 19 | 19 |
| 3 | 95 | 9.5 |
| 4 | 435 | 4.35 |
| 5 | 1853 | 1.85 |
| 6 | 7793 | 0.78 |
| 7 | 32365 | 0.32 |
| 8 | 131200 | 0.13 |
| 9 | 527161 | 0.05 |
| | | |
+-------+------------------------+----------------------------+
The percentage of terms decreases as 10^k increases, and a plausible conjecture is that the asymptotic density of this sequence is 0.
REFERENCES
Richard K. Guy, Unsolved Problems in Theory of Numbers, Springer-Verlag, Third Edition, 2004, B11, p. 102.
LINKS
Marius A. Burtea, Table of n, a(n) for n = 1..10000
EXAMPLE
rad(18)^2 - sigma(18) = (2*3)^2 - (1+2+3+6+9+18) = 36 - 39 = -3 and 18 is a term.
rad(25)^2 - sigma(25) = 5^2 - (1+5+25) = 25 - 31 = -6 and 25 is a term.
rad(40)^2 - sigma(40) = (2*5)^2 - (1+2+4+5+8+10+20+40) = 100 - 90 = 10 and 40 is not a term.
MAPLE
Rad := n -> convert(NumberTheory:-PrimeFactors(n), `*`):
Sigma := n -> NumberTheory:-SumOfDivisors(n):
Is_a := n -> Rad(n)^2 < Sigma(n):
select(Is_a, [`$`(1..500)]); # Peter Luschny, Dec 16 2020
MATHEMATICA
frad2[p_, e_] := p^2; fsig[p_, e_] := (p^(e + 1) - 1)/(p - 1); Select[Range[2, 500], Times @@ frad2 @@@ (f = FactorInteger[#]) < Times @@ fsig @@@ f &] (* Amiram Eldar, Dec 15 2020 *)
PROG
(Magma) s:=func<n|&*PrimeDivisors(n)>; [k:k in [2..500]|s(k)^2 lt DivisorSigma(1, k)]; // Marius A. Burtea, Dec 15 2020
(PARI) isok(k) = factorback(factorint(k)[, 1])^2 < sigma(k); \\ Michel Marcus, Dec 15 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Bernard Schott, Dec 15 2020
EXTENSIONS
More terms from Marius A. Burtea, Dec 15 2020
STATUS
approved