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You are cordially invited to match wits with some of the best minds in IBM Research.

November2025 - Challenge

<< October December >>

We generate a sequence of strings s_0, s_1, s_2, \ldots in the following manner:
  • s_0 = "CAT"
  • s_n is obtained from s_{n-1} by substituting each letter of s_{n-1} according to the following rules: 
    G -> T
T -> CA
C -> TG
A -> C
    The first few steps yield the strings 
    CAT
TGCCA
CATTGTGC
TGCCACATCATTG
    And so forth. One can see from the list that s_{3}[3..7] is "CACA" (here [a..b] means "all the letters, starting from the index a, up to and not including the index b, where indexing starts from 0")

    Your goal: Find s_{n}[n..(n+1000)] for n=10^{100}

    A bonus "*" will be given for finding t_{n}[n..(n+1000)] for n=10^{100} where the sequence t is defined similarly to s but with the initial value t_0 = "RABBITS" and the rules 

    G -> T,
T -> CA,
C -> BR,
A -> I,
R -> B,
B -> IS,
I -> TG,
S -> C,

    We will post the names of those who submit a correct, original solution! If you don't want your name posted then please include such a statement in your submission!

    We invite visitors to our website to submit an elegant solution. Send your submission to the ponder@il.ibm.com.

    If you have any problems you think we might enjoy, please send them in. All replies should be sent to: ponder@il.ibm.com

    •  

    Challenge: 28/10/2025 @ 16:00 PM EST
    Solution: 05/12/2025 @ 12:00 AM EST
    List Updated: 16/12/2025 @ 15:15 PM EST

    *Lazar Ilic(29/10/2025 1:26 PM IDT)
    *Paul Lupascu(29/10/2025 4:27 PM IDT)
    *Bert Dobbelaere(29/10/2025 9:35 PM IDT)
    *Bertram Felgenhauer(29/10/2025 10:53 PM IDT)
    *Prashant Wankhede(29/10/2025 11:27 PM IDT)
    Dan Dima(29/10/2025 11:27 PM IDT)
    *King Pig(30/10/2025 5:42 AM IDT)
    *Guangxi Liu(30/10/2025 9:02 AM IDT)
    *Kang Jin Cho(30/10/2025 2:29 PM IDT)
    *Loukas Sidiropoulos(30/10/2025 7:47 PM IDT)
    *Vladimir Volevich(31/10/2025 10:37 AM IDT)
    Rudy Cortembert(31/10/2025 2:38 PM IDT)
    Juergen Koehl(31/10/2025 7:56 PM IDT)
    *Nadir S.(31/10/2025 11:03 PM IDT)
    *Shirish Chinchalkar(1/11/2025 12:36 AM IDT)
    *Ankit Chakraborty(1/11/2025 4:44 PM IDT)
    Reiner Martin(1/11/2025 5:55 PM IDT)
    *Kevin Cao(1/11/2025 6:08 PM IDT)
    Gary M. Gerken(2/11/2025 6:25 AM IDT)
    *Stephen Ebert(2/11/2025 11:07 AM IDT)
    Aayaam Panigrahi(2/11/2025 2:07 PM IDT)
    *John Tromp(2/11/2025 5:57 PM IDT)
    *Alper Halbutogullari(3/11/2025 8:23 AM IDT)
    *Manish Kumar(3/11/2025 8:29 AM IDT)
    *Latchezar Christov(3/11/2025 2:35 PM IDT)
    *Soonho You(4/11/2025 8:31 AM IDT)
    *Dillon Davis(4/11/2025 9:49 AM IDT)
    *Elias Krainer(4/11/2025 11:32 AM IDT)
    *Karl-Heinz Hofmann(4/11/2025 11:54 AM IDT)
    *Asad Raza(4/11/2025 8:33 PM IDT)
    *Martin Thorne(4/11/2025 8:42 PM IDT)
    Savitha Viswanadh Kandala(5/11/2025 3:57 PM IDT)
    *Ganor Tamir & Shouky Dan(5/11/2025 6:40 PM IDT)
    *Abraham AJ Arshad(5/11/2025 7:01 PM IDT)
    Sathi Gnaneswara Reddy(6/11/2025 4:02 PM IDT)
    *Jiri Spitalsky(7/11/2025 9:26 AM IDT)
    *Ignasi Guillén(7/11/2025 12:06 PM IDT)
    *Daniel Bitin(7/11/2025 5:29 PM IDT)
    *Peter Moser(8/11/2025 5:24 PM IDT)
    Lavanya Kannan(9/11/2025 9:07 PM IDT)
    Reda Kebbaj(10/11/2025 1:57 PM IDT)
    Alex Fleischer(10/11/2025 4:04 PM IDT)
    Steffen Wolf(10/11/2025 6:36 PM IDT)
    Suus Brommer(10/11/2025 10:05 PM IDT)
    *Asaf Zimmerman(10/11/2025 10:09 PM IDT)
    *Dieter Beckerle(11/11/2025 4:54 PM IDT)
    *Nyles Heise(12/11/2025 5:44 AM IDT)
    *Jordan Payette(12/11/2025 10:45 PM IDT)
    *Dominik Reichl(13/11/2025 8:56 PM IDT)
    Hakan Summakoğlu(14/11/2025 8:41 PM IDT)
    *Eran Vered(15/11/2025 10:41 AM IDT)
    *Zachary Barbanell(16/11/2025 12:53 PM IDT)
    *Daniel Chong Jyh Tar(17/11/2025 12:16 AM IDT)
    *Hubert Puszklewicz(17/11/2025 8:49 AM IDT)
    Romeo III Manzanilla(19/11/2025 11:41 AM IDT)
    Morry Rampel Sami(19/11/2025 2:15 PM IDT)
    *Govind Raj Jujare & Hansraj Nahata(19/11/2025 9:02 PM IDT)
    *James Pepper(19/11/2025 11:24 PM IDT)
    Robert Berec(20/11/2025 6:19 PM IDT)
    *Marijonas Kaunas(20/11/2025 9:11 PM IDT)
    Fabio Michele Negroni(21/11/2025 12:08 PM IDT)
    Kandala Srinivas(21/11/2025 7:26 PM IDT)
    Divy Soni(21/11/2025 9:09 PM IDT)
    David Keyes(22/11/2025 11:06 AM IDT)
    Lorenz Reichel(22/11/2025 6:04 PM IDT)
    *Jan Fricke(22/11/2025 7:22 PM IDT)
    Hyung Sik Hwang (huehuehue)(25/11/2025 1:55 AM IDT)
    *Erik Hostens(25/11/2025 1:00 PM IDT)
    Stéphane Higueret(25/11/2025 4:13 PM IDT)
    *Yug Nakrani(26/11/2025 5:17 AM IDT)
    Amos Guler(27/11/2025 12:25 PM IDT)
    Ananda Raidu(27/11/2025 2:04 PM IDT)
    *Hongcheng Zhu(28/11/2025 7:48 PM IDT)
    Todd Will(28/11/2025 10:08 PM IDT)
    *Ankit Aggarwal(28/11/2025 11:28 PM IDT)
    Tim Walters(29/11/2025 4:05 AM IDT)
    Ahmet Yuksel(29/11/2025 9:50 AM IDT)
    *Motty Porat(30/11/2025 1:04 AM IDT)
    David Greer(30/11/2025 1:34 AM IDT)
    *Naftali Peles(30/11/2025 5:13 AM IDT)
    *Chris Shannon(30/11/2025 8:16 AM IDT)
    Kandala Viswakanth(30/11/2025 3:37 PM IDT)
    *David F.H. Dunkley(1/12/2025 2:53 AM IDT)
    Paul Revenant(1/12/2025 9:30 AM IDT)
    Stefan Wirth(1/12/2025 11:46 PM IDT)
    *Jackson La Vallee(3/12/2025 9:26 AM IDT)
    *Li Li(3/12/2025 8:31 PM IDT)
    *Erik Wünstel(4/12/2025 5:03 PM IDT)
    Zoltan Haindrich(4/12/2025 5:24 PM IDT)
    Karl D’Souza(4/12/2025 6:51 PM IDT)
    *Sanandan Swaminathan(4/12/2025 8:17 PM IDT)
    Wyatt Daviau(4/12/2025 8:37 PM IDT)
    Marco Bellocchi(5/12/2025 1:30 AM IDT)
    Karl Mahlburg(5/12/2025 5:22 AM IDT)