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A283075 Number of time intervals that can be measured with n ropes and a lighter from the moment when the first rope begins burning. 1
2, 5, 11, 23, 48, 101, 218, 473, 1044, 2284, 5011, 10979, 23954 (list; graph; refs; listen; history; text; internal format)



Suppose you have n ropes and a lighter. Each rope burns at a nonconstant rate but takes exactly one hour to burn completely from one end to the other. You can only light the ropes at either of their ends but can decide when to light each end as you see fit. If you're strategic in how you burn the ropes, how many specific lengths of time can you measure? (For example, if you had one rope, you could measure two lengths of time: one hour, by simply burning the entire rope from one end, and half an hour, by burning the rope from both ends and marking when the flames meet.)

In this sequence, the time intervals begin when the first rope (or safety fuse, or match cord) begins burning. Without this restriction, if we are allowed to burn some ropes beforehand, more time intervals are available for measuring off. - Andrey Zabolotskiy, Feb 28 2017


Table of n, a(n) for n=1..13.

FiveThirtyEight, The Riddler (see section Riddler Classic), see also the solution (for a(4))

Samuel Cormier-Iijima, Python implementation


a(2)=5: (i) Generate 1 by burning one rope from one end. (ii) Generate 2 by burning one rope from one end at t=0 and the other afterwards at t=1 from one end. (iii) Generate 1/2 by burning 1 rope from both ends. (iv) Generate 3/2 by burning 1 rope from one end at t=0 then the other from both ends at t=1 (or swapped order). (v) Generate 3/4 by burning one rope at t=0 from both ends, starting the other also at t=0 at one end, and lighting the other's second end at t=1/2 when the first rope's flames have reached the middle, so the 2nd rope's flames finish at t=3/4.

For n = 3 the a(3) = 11 solutions are 1/2, 3/4, 7/8, 1, 9/8, 5/4, 3/2, 7/4, 2, 5/2, 3.


Cf. A287012.

Sequence in context: A126017 A292937 A034468 * A130668 A083380 A018112

Adjacent sequences:  A283072 A283073 A283074 * A283076 A283077 A283078




Samuel Cormier-Iijima, Feb 28 2017


a(9) from Andrey Zabolotskiy, Mar 02 2017

a(10)-a(13) from Mark Rickert, Apr 10 2017

Name clarified by Mark Rickert, Jun 08 2017



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Last modified February 20 19:41 EST 2020. Contains 332084 sequences. (Running on oeis4.)