Ciro Santilli @cirosantilli 37

Every undecidable problem must obviously have an infinite number of "possibilities of stuff you can try": if there is only a finite number, then you can brute-force it.

Some undecidable problems are of recursively enumerable language, e.g. the halting problem.

Lists of undecidable problems.

Coolest ones besides the obvious boring halting problem:

mortal matrix problem
Diophantine equation existence of solutions: undecidable Diophantine equation problems

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bitcoin.org Updated 2025-07-16

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bitcoin.org

Official Bitcoin domain registered by Satoshi Nakamoto.

Registration: 2008-08-18 by www.namecheap.com, an American company. But using a privacy oriented registrar: bitcoin.stackexchange.com/questions/89532/how-did-nakamoto-untraceably-pay-for-registering-bitcoin-org It is unknown how he could have paid anonymously, so it seems likely that the true identity could be obtained by law enforcement if needed.

First archive 2009-01-31: web.archive.org/web/20090131115053/http://bitcoin.org/ Also from the archive history web.archive.org/web/20100701000000*/bitcoin.org, things really started picking up on July 2010. This is almost certainly due to the opening of

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Encyclopedia Britannica Updated 2025-07-16

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Ciro Santilli is old enough to remember his parents whispering its name with a respectful tone.

Genius: Richard Feynman and Modern Physics by James Gleick (1994) mentions several times how Richard Feynman was a reader of the encyclopedia. E.g. in youtu.be/ivxkd98mDvc?t=50 Richard's sister also talks about it.

Then the Internet came along and killed it.

The motivation model for collaborators was simple: to get famous. To be able to be selected contribute an article meant that you knew something or two! There was some physicist Ciro read the biography of who was really glad to be able to write an article on the encyclopedia after having worshiped it for so long, TODO find the reference.

While this is somewhat a part of Wikipedia motivation, it is much less so because there is no single article authorship. This is something OurBigBook.com aims to improve.

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Enriched uranium Updated 2025-07-16

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Entrepreneurship in the United Kingdom Updated 2025-07-16

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Startup lists:

Deep tech (have labs) unicorns:

Almost deep tech but possibly without labs:

Exscientia

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Multivariate polynomial Updated 2025-07-16

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A polynomial with multiple input arguments, e.g. with two inputs

x

and

y

f (x, y) = x^{2} + 2 x + y^{3} + 1

(1)

as opposed to a polynomial with a single argument e.g. one with just

x

f (x) = x^{2} + 2 x + 1

(2)

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Phylogenetic tree Updated 2025-07-16

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It is important to note that due to horizontal gene transfer, the early days of life, and still bacteria to this day due to bacterial conjugation, are actually a graph and not a tree, see also: Figure "Graph of life".

Definitely have a look at: coral of life representations.

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Underlying field of a vector space Updated 2025-07-16

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Every vector space is defined over a field.

E.g. in

R^{3}

, the underlying field is

R

, the real numbers. And in

C^{2}

the underlying field is

C

, the complex numbers.

Any field can be used, including finite field. But the underlying thing has to be a field, because the definitions of a vector need all field properties to hold to make sense.

Elements of the underlying field of a vector space are known as scalar.

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UNION (SQL) Updated 2025-07-16

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Basic example tested on SQLite 3.40.1, Ubuntu 23.04:

sqlite3 :memory: 'select 1 union select 2'

output:

1
2

Two columns two rows:

sqlite3 :memory: <<EOF
select * from (values (1, 2), (2, 3))
union
select * from (values (2, 3), (3, 4))
EOF

output:

1|2
2|3
3|4

Note how duplicates are removed, to keep them we UNION ALL instead:

sqlite3 :memory: <<EOF
select * from (values (1, 2), (2, 3))
union all
select * from (values (2, 3), (3, 4))
EOF

output:

1|2
2|3
2|3
3|4

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