Ciro Santilli @cirosantilli 35

Therefore "there aren't that many group homomorphism", and a normal subgroup it is a concrete and natural way to uniquely represent that homomorphism.

The best way to think about the, is to always think first: what is the homomorphism? And then work out everything else from there.

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Busy beaver  Updated 2025-04-24  +Created 1970-01-01

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The busy beaver game consists in finding, for a given

n

, the turing machine with

n

states that writes the largest possible number of 1's on a tape initially filled with 0's. In other words, computing the busy beaver function for a given

n

There are only finitely many Turing machines with

n

states, so we are certain that there exists such a maximum. Computing the Busy beaver function for a given

n

then comes down to solving the halting problem for every single machine with

n

states.

Some variant definitions define it as the number of time steps taken by the machine instead. Wikipedia talks about their relationship, but no patience right now.

The Busy Beaver problem is cool because it puts the halting problem in a more precise numerical light, e.g.:

the Busy beaver function is the most obvious uncomputable function one can come up with starting from the halting problem
the Busy beaver scale allows us to gauge the difficulty of proving certain (yet unproven!) mathematical conjectures

Bibliography:

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Room temperature superconductor  Updated 2025-04-24  +Created 1970-01-01

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Stack Overflow maximum 200 daily reputation limit  Updated 2025-04-24  +Created 1970-01-01

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Why. Why. Why is there no limit to how much I can help, but there is a limit to how many thanks I can get?

At most, limit it to a single answer to avoid highly publicized events, e.g. an answer being shared on Reddit. But across answers? It makes no sense.

The two ways main ways to overcome this limit are the 15 point answer accept reputation and bounties.

200 reputation per day works out 73k a year BTW.

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Stack Overflow policy that you cannot harshly criticize the character of a politician  Updated 2025-04-24  +Created 1970-01-01

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cirosantilli.com/china-dictatorship/stack-overflow-forbids-criticizing-the-character-of-genocidal-political-leaders-like-xi-jinping

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Television series  Updated 2025-04-24  +Created 1970-01-01

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Create a test user in PostgreSQL  Updated 2025-04-24  +Created 1970-01-01

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In order to create a test user with password instead of peer authentication, let's create test user:

createuser -P user0
createdb user0

-P makes it prompt for the users password.

Alternatively, to create the password non-interactively stackoverflow.com/questions/42419559/postgres-createuser-with-password-from-terminal:

psql -c "create role NewRole with login password 'secret'"

Can't find a way using the createuser helper.

We can then login with that password with:

psql -U user0 -h localhost

which asks for the password we've just set, because the -h option turns off peer authentication, and turns off password authentication.

The password can be given non-interactively as shown at stackoverflow.com/questions/6405127/how-do-i-specify-a-password-to-psql-non-interactively with the PGPASSWORD environment variable:

PGPASSWORD=a psql -U user0 -h localhost

Now let's create a test database which user0 can access with an existing superuser account:

createdb user0db0
psql -c 'GRANT ALL PRIVILEGES ON DATABASE user0db0 TO user0'

We can check this permission with:

psql -c '\l'

which now contains:

                                  List of databases
   Name    |  Owner   | Encoding |   Collate   |    Ctype    |   Access privileges
-----------+----------+----------+-------------+-------------+-----------------------
 user0db0  | ciro     | UTF8     | en_GB.UTF-8 | en_GB.UTF-8 | =Tc/ciro             +
           |          |          |             |             | ciro=CTc/ciro        +
           |          |          |             |             | user0=CTc/ciro

The permission letters are explained at:

user0 can now do the usual table operations on that table:

PGPASSWORD=a psql -U user0 -h localhost user0db0 -c 'CREATE TABLE table0 (int0 INT, char0 CHAR(16));'
PGPASSWORD=a psql -U user0 -h localhost user0db0 -c "INSERT INTO table0 (int0, char0) VALUES (2, 'two'), (3, 'three'), (5, 'five'), (7, 'seven');"
PGPASSWORD=a psql -U user0 -h localhost user0db0 -c 'SELECT * FROM table0;'

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