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Let $a_{n}=$ the first $p_{n}$th digits to the right of decimal point of the square root of the $n$th prime.

Example:

$\sqrt{2}=1.414213562...$

So, $a_{1}=41$

$\sqrt{3}=1.73205080...$

So, $a_{2}=732$

$\sqrt{5}=2.236067977499789696409173668731276...$

So, $a_{3}=23606$

$\sqrt{7}=2.64575131106459059050161575363926...$

So, $a_{4}=6457513$

$\sqrt{11}=3.316624790355399849114932736...$

So, $a_{5}=31662479035$

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The number I want to construct:

$0.a_{1}a_{2}a_{3}a_{4}a_{5}...=0.4173223606645751331662479035...$

This number exists by monotone bounded sequence theorem applied to the sequence of partial sums of the series forming the number.

Could I say that this constant is transcendental?

Update at $12$:$21$AM of May/$31$/$2024$:

I have added the following summation formula for the number:

$0.a_{1}a_{2}a_{3}a_{4}a_{5}...=\sum_{n=1}^{\infty} a_{n}10^{- \sum_{j=1}^{n} p^{j}}=\sum_{n=1}^{\infty} \lfloor10^{p_{n}}(\sqrt{p_{n}}-\lfloor \sqrt{p_{n}} \rfloor) \rfloor 10^{- \sum_{j=1}^{n} p^{j}} $

Math Admiral
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1 Answers1

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I'd like to propose a probabilistic argument for the following:

$M=0.a_{1}a_{2}a_{3}...$ is impossible to be periodic, which would mean it is irrational.

Assume $M$ is periodic, so $M$ has period length equaling $m \in \mathbb{N}$.

Let $k_{1}k_{2}...k_{m}$ be the repeating block of digits in $M$.

By definition of $a_{n}$, there exists sufficiently large $s$ such that $a_{s} \geq2m>m$.

This means that we would have the block $k_{1}k_{2}...k_{m}$ is contained somewhere in $a_{n}$ for all square roots of primes $p_{n}$ at least one time when $n \geq s$.

I think this is unlikely to happen.

Math Admiral
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  • I would greatly appreciate the feedback of the experts on this site, and welcome any suggestions to improve my answer based on your knowledge and expertise. – Math Admiral May 31 '24 at 22:01
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    I see no argument for why what you claim is "unlikely" really is unlikely. Can you imagine it false for $m=1$? For $m=2$? What value of $m$ would do? – Ethan Bolker May 31 '24 at 22:16
  • @EthanBolker This $m$ would be large if existed, so my argument is it possible that the repeating block would appear forever in $a_{n}$, for all $n \geq s$? – Math Admiral May 31 '24 at 22:18
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    Sure it's possible for any particular block of size $m$. You need an actual argument to show it doesn't happen, or a good estimate to show it happens with very low probability. Just thinking it's unlikely doesn't make it so. – Ethan Bolker May 31 '24 at 22:25
  • Video containing similar ideas: https://youtu.be/QjF8Uip9AYA?si=jS_DDUMfsvXSNXuN – Math Admiral Jun 02 '24 at 16:17
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    You say it's impossible that it is periodic, but that's difficult to prove. It could be that the square roots of primes all have a bunch of 3s in their decimal expansions at what just so happens to be the exact right place for this number to end in just all 3s. Or any other kind of repeating pattern. Unlikely, yes. Extremely. But possible. – Arthur Jun 07 '24 at 11:38