Problem 52859. Easy Sequences 35: Cutting a donut to Semiprime pieces
Solution Stats
Problem Comments

3 Comments
For n>330289
something weird happens in this series...
Many more numbers pop up as part of the solution.
For example:
330290 ## 463 * 12970537638763 = 6005358926747269
330321 ## 165161 * 36370874563 = 6007050013699643
330326 ## 2678131 * 2243102671 = 6007322799387901
These 3 examples are semiprimes.
You can check in MATLAB command window:
factor( C(330290) )
factor( C(330321) )
factor( C(330326) )
where C(n) is the equation for the cutspieces on a torus.
For this reason, the test 6 of this problem may be wrong and should updated for x=3e5.
Actually, factor(C(330290)) would return:
[ 2 5 23 29 33029 27259489]
without rounding errors.
Flintmax ~ 9e15, so if you calculated ~6e15 as n/6, n was clearly above flintmax.
A key point to solve this quickly for large inputs is to notice that any prime factors of n other than 2 or 3 will necessarily be factors of C(n) because the polynomial has a zero constant term.
@GeeTwo
thank you for pointing out a crucial detail that I was missing!
Solution Comments
Show commentsProblem Recent Solvers6
Suggested Problems

Read a column of numbers and interpolate missing data
2171 Solvers

middleAsColumn: Return all but first and last element as a column vector
594 Solvers

161 Solvers

Construct the SeidelEntringerArnold triangle
21 Solvers

86 Solvers
More from this Author116
Problem Tags
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!