# Looking for reading material on: Numbers, in whose internal decimal places appear all natural numbers as a sequence of digits

For example one can have the number 0.12 and can look at the sequence of the digits of it's internal decimal places and see, that 0.12 contains the numbers 1,2 and 12. It is also easy to construct a number, that contain all natural numbers by just numerating the natural numbers and write them one after the other behind the 0. . However I was wondering, if it were possible to find all natural numbers in beautiful numbers like Pi or e or $\sqrt2$ etc. , also if this is some sort of property only a few numbers share.

Can anyone recommend some sort of reading material on that topic or knows more about it? As always: Many thanks in advance.

• Since you are asking for a recommendation: If you are interested in serious mathematics I'd recommend that you abandon numerology connected with the decimal system and go on to rejoice in real mathematical content. Commented May 30, 2015 at 20:37
• See related question: math.stackexchange.com/questions/216343/… for the question of if $\pi$ is normal. Commented May 30, 2015 at 20:39
• Also of importance to note: the weaker property "contains all natural numbers" you describe is a disjunctive number, as opposed to a normal number where all substrings of digits of the same length must appear with the same frequency. Commented May 30, 2015 at 20:44
• @ChristianBlatter: The only connection I see between numerology and my post is, that I find some numbers more beautiful than others and that I find it far from obvious that there are numbers that fulfill that given property. Commented May 30, 2015 at 20:47
• wow, thanks to JVMoravitz, user2566092 I guess this material is enough and should keep me occupied for quite some time. I didn't know those numbers were actually called "normal" - otherwise I had been able to just look it up. Thanks again. Commented May 30, 2015 at 20:52

It is a very hard thing to prove that any particular number is Normal, unless it has been particularly constructed with a proof in mind. I believe the cases of $e$ and $\pi$ and $\sqrt 2$ are open.
This has been asked before on MathOverflow and as far as I know based on the responses there, there is no known "typical" irrational number like $\sqrt{2}$ or $\pi$ which is known to contain all natural numbers as some sequence of consecutive digits. Note there are uncountably many irrational numbers that do NOT have this property, e.g. all the irrational numbers that do not contain a certain digit in their expansion.