Number Gossip

(Enter a number and I'll tell you everything you wanted to know about it but were afraid to ask.)

Unique Properties of 2491

2491 is the largest number with two prime factors that together sum to 100
2491 is the smallest sum of the sides of a tetrahedron consisting of Pythagorean triangles

Common Properties of 2491

Apocalyptic power

The number n is called an apocalyptic power if 2ⁿ contains the consecutive digits 666 (in decimal).

...,
2487,
2489,
2490,
2491,
2492,
2493,
2500,
...

Composite

A positive integer greater than 1 that is not prime is called composite.

Composite numbers are opposite to prime numbers.

...,
2488,
2489,
2490,
2491,
2492,
2493,
2494,
...

Deficient

The number n is deficient if the sum of all its positive divisors except itself is less than n.

Compare with perfect and abundant numbers.

...,
2487,
2488,
2489,
2491,
2493,
2494,
2495,
...

Evil

The number n is evil if it has an even number of 1's in its binary expansion.

Guess what odious numbers are.

...,
2484,
2487,
2488,
2491,
2493,
2494,
2496,
...

Lucky

To build the lucky number sequence, start with natural numbers. Delete every second number, leaving 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, ... . The second number remaining is 3, so delete every third number, leaving 1, 3, 7, 9, 13, 15, 19, 21, ... . The next number remaining is 7, so delete every 7th number, leaving 1, 3, 7, 9, 13, 15, 21, ... . The next number remaining is 9, so delete every ninth number, etc.

Those numbers were lucky they weren't crossed out.

...,
2467,
2473,
2479,
2491,
2493,
2505,
2511,
...

Odd

A number is odd if it is not divisible by 2.

Numbers that are not odd are even. Compare with another pair -- evil and odious numbers.

...,
2485,
2487,
2489,
2491,
2493,
2495,
2497,
...

Square-free

A number is said to be square-free if its prime decomposition contains no repeated factors.

...,
2487,
2489,
2490,
2491,
2494,
2495,
2497,
...

Tanya Khovanova

Number Gossip

Unique Properties of 2491

Common Properties of 2491