OFFSET
1,2
COMMENTS
These integers are refactorable: the number of divisors divides the number itself.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..388
Simon Colton, Refactorable Numbers - A Machine Invention, J. Integer Sequences, Vol. 2 (1999), Article 99.1.2.
Simon Colton, HR - Automatic Theory Formation in Pure Mathematics. [Wayback Machine copy]
FORMULA
(p_1)^(m_1)...(p_k)^(m_k) -> (p_1)^((p_1)^(m_1) - 1)...(p_k)((p_k)^(m_k) - 1).
EXAMPLE
a(6) = 18 because 6 = 2^(1)3^(1) -> 2^(2^(1) - 1)3^(3^(1) - 1) = 18.
MATHEMATICA
f[p_, e_] := p^(p^e-1); a[1] = 1; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 24] (* Amiram Eldar, Sep 24 2023 *)
PROG
(PARI) a(n) = my(f = factor(n)); for (i=1, #f~, f[i, 2] = f[i, 1]^f[i, 2] - 1); factorback(f); \\ Michel Marcus, Dec 08 2014
CROSSREFS
KEYWORD
nonn,mult
AUTHOR
Simon Colton (simonco(AT)cs.york.ac.uk)
STATUS
approved