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Omni-MATH / To every natural number k,k≥2, there corresponds a sequence a_(n)(k) according to the following rule: a₀=k, a_(n)=τ(a_(n−1)) ∀n≥1, in which τ(a) is the number of different divisors…

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problem

To every natural number k,k≥2k, k \geq 2, there corresponds a sequence an(k)a_n(k) according to the following rule: a0=k,an=τ(an−1)∀n≥1,a_0 = k, \qquad a_n = \tau(a_{n-1}) \quad \forall n \geq 1, in which τ(a)\tau(a) is the number of different divisors of aa. Find all kk for which the sequence an(k)a_n(k) does not contain the square of an integer.
Plain-text mathematical notation (without MathML)
To every natural number k,k≥2, there corresponds a sequence a_(n)(k) according to the following rule:
a₀=k, a_(n)=τ(a_(n−1)) ∀n≥1,
in which τ(a) is the number of different divisors of a. Find all k for which the sequence a_(n)(k) does not contain the square of an integer.
Original LaTeX notation
To every natural number $k, k \geq 2$, there corresponds a sequence $a_n(k)$ according to the following rule:
\[a_0 = k, \qquad a_n = \tau(a_{n-1})  \quad \forall n \geq 1,\]
in which $\tau(a)$ is the number of different divisors of $a$. Find all $k$ for which the sequence $a_n(k)$ does not contain the square of an integer.

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