A353890 a(n) is the period of the binary sequence {b(m)} defined by b(m) = 1 if (m+1)^n - m^n and (m+2)^n - 2*(m+1)^n + m^n are coprime, 0 otherwise.
1, 1, 5, 11, 91, 1247, 3485, 263017, 852841, 1241058127, 74966255, 243641132605417, 181556731572385303, 718802057694183783881, 6582662048285, 943422576750791493013356207217, 487331778345355477261, 607088607861933740557075591887834842297
Offset: 2
Keywords
Examples
For n=2 and n=3, the first and second differences are coprime for all m. Each of their sequences {b(m)} consist only of 1's, which can be described trivially as [1] with a period of 1, so a(2) = a(3) = 1. For n > 3, the first and second differences are coprime for some m values, but not for all. Each repeating periodic sequence {b(m)} begins at m=1, and can be used to predict what b(m) will be at any higher m value for that power n. n=4 has the 5-term repeating sequence, beginning at m=1: [0 0 1 1 1], so a(4) = 5. The sequence is repeating, so for example, f(41)..f(45) is also [0 0 1 1 1]. n=5 has the 11-term repeating sequence [1 1 0 1 1 0 1 1 1 1 1] so a(5) = 11. n=6 has the 91-term repeating sequence [0 0 0 0 0 0 1 0 0 0 0 1 1 1 0 0 0 0 0 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 1 0 0 0 0 1 1 0 0 0 0 0 1 1 1 0 0 0 0 1 0 0 0 0 0 0 1 1 1] so a(6) = 91. The period for higher n values has yet to be found. If they exist, it seems they would be quite large given the large expansion from 5, 11, to 91. Example: the 233rd term in the sequence of values for n=6 is calculated by using m=233 and n=6. Define the first difference for the 233rd term as 234^6 - 233^6 = 4164782373647. The second difference for the 233rd term is 235^6 - 2*234^6 + 233^6 = 89948228762. The terms 4164782373647 and 89948228762 share a common factor, so the 233rd term of the sequence for 6th powered terms is denoted 0 (not coprime). Because the 6th powered terms repeat their tendency of being coprime or not every 91 terms, we could instead look at 233 mod 91 = 51, and from the table for n=6 above, the 51st term is 0.
Links
- Samuel Harkness, MATLAB program
Crossrefs
Programs
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MATLAB
See Links section.
Extensions
a(7)-a(19) from Jon E. Schoenfield, May 10 2022
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