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Let $k -$ positive integer, $k \ge2$. The sequence $a_n(k)$ such that $$a_0(k)=k, a_n(k)=\tau\left(a_{n-1}(k)\right),$$ for $n\ge1$, where $\tau(a) -$ the number different divisors of the number $a$.

Find all $k$, such that the sequence $a_n(k)$ does not contain squares of positive integers.

My work so far:

If $a=p_1^{\alpha_1}\cdot p_2^{\alpha_2}\cdot...\cdot p_n^{\alpha_n}$ then $$\tau(a)=(\alpha_1+1)(\alpha_2+1)\cdot...\cdot(\alpha_n+1)$$

Let $k=2$ $$a_1=2, a_2=2,a_3=2,...$$ Hence, $a_n(2) -$ satisfies.

Let $k=6$ $$a_1=6, a_2=4=\color{red}{2^2}, $$ Hence, $a_n(6) -$ not satisfies.

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Those $k$ are precisely the prime numbers.

Because $2\leq\tau(n)<n$ for $n\geq3$, every sequence eventually reaches $2$, from which point it is constant.
Let $k$ be given and suppose $n$ is the smallest index with $a_n=2$. Suppose $n\geq2$.
Then $a_{n-1}$ has $2$ divisors, and is thus prime.
Then $a_{n-2}$ is a prime power with exponent $a_{n-1}-1$. But by the minimality of $n$, $a_{n-1}\neq2$ so $a_{n-1}$ is odd, which means $a_{n-2}$ is a square.

This reason is invalid if $n\leq1$, i.e. if it takes at most $1$ step to reach $2$, which is the case if and only if $k$ is prime.

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