Hi friend,

The last issue of the newsletter - #90 - included a list of cars with the number 90 in their name.

JC, a very astute reader and self-described “part time car enthusiast” sent me a few more cars that also include 90 in their name.

• Volvo XC90 - on the market presently

• BMW E90 - internal BMW numbering for 5th generation 3 series

• Porsche 90 - an iconic Porsche 1961-63 356B Super Coupe...

I find this rather interesting. Why is 90 so prevalent in the names of these cars?

Any thoughts? Speculations?

Stay curious, friend!

Love,

• Harshal

P.S. I am going to be traveling next week so issue #92 will be delayed by a couple of days. You can always get your fix to sate your curiosity in the archives at https://sixstemtweets.beehiiv.com/ - 100+ issues, 600+ fascinating bits of information.

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This is issue #91. Let’s see what makes 91 an interesting number:

• Ninety-one is a version of solitaire, played with a deck of cards. The details are at https://en.wikipedia.org/wiki/Ninety-One_(solitaire) IMO - this doesn’t seem like a very interesting game but if you have played it, let me know.

• 91 is the second semiprime of the form (7.q), where q is a higher prime.

• 91 is the smallest positive integer expressible as a sum of two cubes in two different ways if negative roots are allowed (alternatively the sum of two cubes and the difference of two cubes):
  91 = 6³ + (−5)³ = 4³ + 3³ .
  This implies that 91 is the second cabtaxi number.

• the smallest positive integer expressible as a sum of six distinct squares:
  91 = 1² + 2² + 3² + 4² + 5² + 6² .

• The only other ways to write 91 as a sum of distinct squares are:
  91 = 1² + 4² + 5² + 7² and

• 91 = 1² + 3² + 9² .

• a repeated digit (aka repdigit) in base 9 (111₉).

• palindromic in bases 3 (10101₃), 9 (111₉), and 12 (77₁₂).

• 91 is the octane rating of “premium gasoline” in many countries

• Protactinium is a chemical element; it has symbol Pa and atomic number 91. It is a dense, radioactive, silvery-gray actinide metal which readily reacts with oxygen, water vapor, and inorganic acids.

  • Protactinium is one of the rarest and most expensive naturally occurring elements.

  • The element was first identified in 1913 and named "brevium" because of the short half-life of the specific isotope studied, ^234m Pa. A more stable isotope of protactinium, ²³¹ Pa, was discovered in 1917/18. In 1949, the IUPAC chose the name "protactinium". The new name meant "(nuclear) precursor of actinium,"suggesting that actinium is a product of radioactive decay of protactinium.

• 91 is the international country calling code for India

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

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