Riddler Classic, July 3 2020

Got last week’s Express just right, with the right explanation.

This week’s Classic is based on the US flag, but the maths question is this:

[W]hen N equals 50, N is twice a square and N+1 is a centered pentagonal number. After 50, what is the next integer N with these properties?

This one seems very easy to brute force. My code returns an answer in less than a thousandth of a second. The returned answer is twice a square, and one less than a centred pentagonal. It’s definitely possible that I missed the correct, lower answer because there’s a mistake in my logic for stepping through the possible doubled squares and centred pentagonals.

I’m very curious to know if there’s a good way to solve this analytically.

2 thoughts on “Riddler Classic, July 3 2020

Leave a Reply to cjb230 Cancel reply

Your email address will not be published. Required fields are marked *

I accept that my given data and my IP address is sent to a server in the USA only for the purpose of spam prevention through the Akismet program.More information on Akismet and GDPR.

This site uses Akismet to reduce spam. Learn how your comment data is processed.