Problem 69. Find the peak 3n+1 sequence value

Created by Cody Team

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A Collatz sequence is the sequence where, for a given number n, the next number in the sequence is either n/2 if the number is even or 3n+1 if the number is odd. See Problem 21 for more information.
Let c(n) be the sequence for n, and p(n) be the peak value of that sequence. For a given threshold nmax, find the highest peak value max(p(n)) for all Collatz sequences starting with integers between 1 and nmax.

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Last Solution submitted on Oct 05, 2025

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Hariprasad Baburajan on 25 Dec 2018

Nice one

Zoltán Nagy on 26 Apr 2019

Good problem!

Mark McBroom on 10 Jan 2021

good.

Rik on 13 Jan 2021

If you want to add more tests: https://oeis.org/A006884 and https://oeis.org/A006885 contain the relevant sequences.

Tanzil Parvez (Emon) on 3 May 2021

Why the leading solution size is 0 ? Is it for everyone or just me

goc3 on 3 May 2021

@Tanzil Parvez (Emon): the hack solutions (hopefully all) have been deleted from this problem.

Md. Nayem Uddin Biswas on 8 Aug 2022

Wrong test cases! I checked on internet and found all the test cases except case 1 are wrong.

Dyuman Joshi on 8 Aug 2022

The test cases are all correct. You have simply not understood what the question is asking.

Meg Noah on 14 Oct 2022

Broken link in description.

Dyuman Joshi on 14 Oct 2022

Link has been updated.

xb on 21 Aug 2023

why does it not work it can run successfully in matalb
function pmax = peakOfPeaks(nmax)
B=[]
for n=1:1:nmax
A=[];
if n==1
A=1
else A=n
end
while n~=1
if mod(n,2)==1
A=[A,3*n+1];
n=3*n+1
else A=[A,n/2];
n=n/2
end
end
B=[B,max(A)]
end
pmax=max(B)
end

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Problem 69. Find the peak 3n+1 sequence value

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Problem 69. Find the peak 3n+1 sequence value

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