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Here are five problems which you might like:

(1) Are there 44 unit vectors in $\mathbb{R}^5$ where the dot product between each pair is less than $\frac{1}{2}$?

I originally saw this formulation on this MO post. It is open, and related to the kissing number in 5 dimensions.

(2) A Hadamard Matrix is a square matrix all of whose entries are $\pm1$, and whose rows are mutually orthogonal. Prove that there exists a Hadamard Matrix of size $4k$ for every $k\geq 1$.

This is known as the Hadamard Conjecture. While it was considered by Hadamard in the 19th century, it is still open today.

(3) Given $n$ distinct points in the plane, what is the minimum number of distinct distances between those points?

This is a famous problem of Erdos, and while it has not completely been resolved, a near optimal bound belongs to Guth and Katz. (See Terence Tao's blog post)

(4) Prove that $$\sigma(n)\leq H_n +e^{H_n}\log H_n$$ where $\sigma(n)$ is the sum of divisors function and $H_n=1+\frac{1}{2}+\cdots+\frac{1}{n}$ is the $n^{th}$ Harmonic number.

This problem is equivalent to the Riemann Hypothesis. This reformulation was done by Jeffrey Lagarias. (Since the reformulation was more recent, I think it qualifies for this list)

(5) If $\alpha\neq 0,1$ is algebraic, and $\beta$ is irrational algebraic, will $\alpha^\beta$ be transcendental?

This is the very well known Hilbert's 7th Problem. This was resolved by Gelfond and Schneider in 1934. There Related to this, there is the very famous story of Hilbert giving a lecture in 1919 where he said that he might see the proof of the Riemann hypothesis in his life time, that the youngest members of the audience might live to see Fermatâ€™s Last Theorem proved, but no one present in the hall would live to see a proof of transcendence of $2^{\sqrt{2}}$. Of course the exact opposite happened.

Here are five problems which you might like:

(1) Are there 44 unit vectors in $\mathbb{R}^5$ where the dot product between each pair is less than $\frac{1}{2}$?

I originally saw this formulation on this MO post. It is open, and related to the kissing number in 5 dimensions.

(2) A Hadamard Matrix is a square matrix all of whose entries are $\pm1$, and whose rows are mutually orthogonal. Prove that there exists a Hadamard Matrix of size $4k$ for every $k\geq 1$.

This is known as the Hadamard Conjecture. While it was considered by Hadamard in the 19th century, it is still open today.

(3) Given $n$ distinct points in the plane, what is the minimum number of distinct distances between those points?

This is a famous problem of Erdos, and while it has not completely been resolved, a near optimal bound belongs to Guth and Katz. (See Terence Tao's blog post)

(4) Prove that $$\sigma(n)\leq H_n +e^{H_n}\log H_n$$ where $\sigma(n)$ is the sum of divisors function and $H_n=1+\frac{1}{2}+\cdots+\frac{1}{n}$ is the $n^{th}$ Harmonic number.

This problem is equivalent to the Riemann Hypothesis. This reformulation was done by Jeffrey Lagarias. (Since the reformulation was more recent, I think it qualifies for this list)

(5) If $\alpha\neq 0,1$ is algebraic, and $\beta$ is irrational algebraic, will $\alpha^\beta$ be transcendental?

This is the very well known Hilbert's 7th Problem. This was resolved by Gelfond and Schneider in 1934. There is the very famous story of Hilbert giving a lecture in 1919 where he said that he might see the proof of the Riemann hypothesis in his life time, that the youngest members of the audience might live to see Fermatâ€™s Last Theorem proved, but no one present in the hall would live to see a proof of transcendence of $2^{\sqrt{2}}$. Of course the exact opposite happened.

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Here are five problems which you might like:

(1) Are there 44 unit vectors in $\mathbb{R}^5$ where the dot product between each pair is less than $\frac{1}{2}$?

I originally saw this formulation on this MO post. It is open, and related to the kissing number in 5 dimensions.

(2) A Hadamard Matrix is a square matrix all of whose entries are $\pm1$, and whose rows are mutually orthogonal. Prove that there exists a Hadamard Matrix of size $4k$ for every $k\geq 1$.

This is known as the Hadamard Conjecture. While it was considered by Hadamard in the 19th century, it is still open today.

(3) Given $n$ distinct points in the plane, what is the minimum number of distinct distances between those points?

This is a famous problem of Erdos, and while it has not completely been resolved, a near optimal bound belongs to Guth and Katz. (See Terence Tao's blog post)

(4) Prove that $$\sigma(n)\leq H_n +e^{H_n}\log H_n$$ where $\sigma(n)$ is the sum of divisors function and $H_n=1+\frac{1}{2}+\cdots+\frac{1}{n}$ is the $n^{th}$ Harmonic number.

This problem is equivalent to the Riemann Hypothesis. While the Riemann hypothesis was proposed in the 19th century, and while it is number theory, I think this This reformulation merited a place on this list. It was done by Jeffrey Lagariasin . (Since the 20th centuryreformulation was more recent, so that should make I think it qualify.qualifies for this list)

(5) If $\alpha\neq 0,1$ is algebraic, and $\beta$ is irrational algebraic, will $\alpha^\beta$ be transcendental?

This is the very well known Hilbert's 7th Problem.

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