Number Gossip

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Unique Properties of 651

651 is the smallest non-trivial nonagonal pentagonal number
651 is the smallest number whose base-5 representation has exactly 5 runs

Rare Properties of 651

Pentagonal

Pentagonal numbers are of the form n(3n - 1)/2.

Pentagonal numbers are to pentagons what triangular numbers are to triangles and square numbers are to squares.

...,
477,
532,
590,
651,
715,
782,
852,
...

Common Properties of 651

Composite

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

Composite numbers are opposite to prime numbers.

...,
648,
649,
650,
651,
652,
654,
655,
...

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.

...,
646,
647,
649,
651,
652,
653,
655,
...

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.

...,
639,
643,
645,
651,
655,
673,
679,
...

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.

...,
645,
647,
649,
651,
653,
655,
657,
...

Odious

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

Guess what evil numbers are.

...,
644,
647,
648,
651,
653,
654,
656,
...

Square-free

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

...,
646,
647,
649,
651,
653,
654,
655,
...

Tanya Khovanova

Number Gossip

Unique Properties of 651

Rare Properties of 651

Common Properties of 651