Hi friends,

Thanks for your feedback and for sharing this newsletter with your fellow curious humans.

There’s lots of wonder and awe in science, math, engineering - let’s help spread it.

To commemorate this issue #90 of the newsletter, here’s a pro-tip: If you ever get cold and don't have a sweater, stand in a corner for a few minutes; they're usually about 90 degrees. (Of course, this joke only works in places that use F-reedom units)

Here’s one more: 90 degrees is pretty hot for most people. But for mathematicians, it's just right. 😁 

Have a great week!

Love,

Harshal

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This is issue #90. Let’s see what makes 90 an interesting number:

• 90 is a pronic number as it is the product of 9 and 10 - A pronic number is a number that is the product of two consecutive integers that is, a number of the form n ( n + 1 ).

  • The study of these numbers dates back to Aristotle. They are also called oblong numbers, heteromecic numbers, or rectangular numbers;

  • Along with 12 and 56 one of only a few pronic numbers whose digits in decimal are also successive.

• 90 is divisible by the sum of its base-ten digits, which makes it the thirty-second Harshad number

  • a Harshad number (or Niven number) in a given number base is an integer that is divisible by the sum of its digits when written in that base

  • Harshad numbers were defined by D. R. Kaprekar, a mathematician from India. The word "harshad" comes from the Sanskrit harṣa (joy) + da (give), meaning joy-giver. The term "Niven number" arose from a paper delivered by Ivan M. Niven at a conference on number theory in 1977.

• An angle measuring 90 degrees is called a right angle. In normal space, the interior angles of a rectangle measure 90 degrees each, while in a right triangle, the angle opposing the hypotenuse measures 90 degrees, with the other two angles adding up to 90 for a total of 180 degrees.

• 90 can be expressed as the sum of distinct non-zero squares in six ways, more than any smaller number

  • ( 9² + 3² ) , ( 8² + 5² + 1² ) , ( 7² + 5² + 4² ) , ( 8² + 4² + 3² + 1² ) , ( 7² + 6² + 2² + 1² ) , ( 6² + 5² + 4² + 3² + 2² )

• 90 is the atomic number of Thorium - a chemical element with symbol Th. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air. It is named for Thor - the Norse god of thunder

• 90 is the international calling code for Turkey

• Interestingly enough, there are many car models with ‘90’ in the name (all links below are to Wikipedia)

  • Audi 90, a compact executive car produced by Audi

  • Saab 90, a compact executive car produced by Saab

  • Sunbeam-Talbot 90, a compact executive car produced by Sunbeam-Talbot

  • Alfa Romeo 90, an executive car produced by Alfa Romeo

  • Tatra 90, a prototype mid-size car

  • Rover 90, a saloon produced by the Rover Company

That’s it for this issue.

Hit ‘reply’ to tell me what you think.

And hit ‘forward’ to share with your friends and family.

Let’s all celebrate science and engineering and curiosity.

Best wishes,

Harshal

Special past issues

• Venn diagrams

• Total solar eclipse

• Aurora (Northern lights)

• Nobel prize - 2024

• Physical Nobel prize - 2024

• Chemistry Nobel prize - 2024

• Halloween 2024

• Fibonacci series

• pi day 2024

• Blue Ghost (Firefly) Lunar lander

• Pope Leo XIV

• Dr. Jayant Narlikar