20180711, 10:59  #1 
Jul 2018
3 Posts 
Fascinating periodic sequence pairs
I am developing a sound synthesizer whose waveforms are based on periodic number sequences, during the course of which I chanced across the following relations, and I'd be grateful for any help in characterizing/formalizing these observations:
Let w(n) = w(n1) + 2*w(n2)  2*w(n3). Let a(n) = w(n) mod m. Observation 1 Set a(n) as the sequence with initial values w(0)=0, w(1)=1, w(2)=m+2. Set a'(n) as the sequence with initial values w(0)=0, w(1)=0, w(2)=m+2. Then if we call a(n) the 'detailed' sequence, a'(n) is the corresponding 'sample and hold'/'coarse'/'envelope following' sequence (not sure what to call it). Example, for m=17: a(n) has initial values w(0)=0, w(1)=1, w(2)=19, and the sequence is: 0,1,2,4,6,10,14,5,13,12,11,9,7,3,16,8,0,1,2,4,6,10,14,5,13,12,11,9... a'(n) has initial values w(0)=0, w(1)=0, w(2)=19, and the sequence is: 0,0,2,2,6,6,14,14,13,13,11,11,7,7,16,16,0,0,2,2,6,6,14,14,13,13,11,11... Here is a graph of these sequences a(n) and a''(n) on the same axes: https://i.imgur.com/jI0Lwta.png Observation 2 For even m: Set a(n) as the sequence with initial values w(0)=0, w(1)=1, w(2)=m/2. Set a'(n) as the sequence with initial values w(0)=0, w(1)=1, w(2)=(m/2)+1. Let a''(n) = a'(n1)+1. Then if a(n) is the 'detailed' sequence, a''(n) is the corresponding 'envelope' sequence. Example for m=34: a(n) has initial values w(0)=0, w(1)=1, w(2)=17, and the sequence is: 0,1,17,19,17,21,17,25,17,33,17,15,17,13,17,9,17,1,17,19,17,21,17,25,17,33,17,15... a'(n) has initial values w(0)=1, w(1)=1, w(2)=18, and the sequence is: 0,1,18,20,20,24,24,32,32,14,14,12,12,8,8,0,0,18,18,20,20,24,24,32,32... Sequence a''(n) is obtained by adding 1 to every value of a'(n) and then shifting all values forward by 1 place: ,1,2,19,21,21,25,25,33,33,15,15,13,13,9,9,1,1,19,19,21,21,25,25,33,33,15,15... Here is a graph of these sequences a(n) and a''(n) on the same axes: https://i.imgur.com/Z4qKJ4u.png The discovery of above recursion formula which yields these interesting 'detailed' and 'envelope' sequences simply by incrementing a couple of the initial values, has provided a very simple way of obtaining additional sonic diversity from the synthesizer 
20180711, 14:36  #2  
Aug 2006
2·3^{2}·5^{2}·13 Posts 
Quote:


20180712, 01:29  #3 
Jul 2018
3_{10} Posts 
Thanks very much. So if we consider the sequences without the mod m and compare with what's currently in OEIS, would you agree there is justification for the following submissions?
1) A027383 is 1,2,4,8,10,14,22,30... a(n) = a(n1) + 2*a(n2)  2*a(n3), with a(0)=0, a(1)=1, a(2)=2 is: 0,1,2,4,,8,10,14,22,30..... Action: Add a new sequence in OEIS for a(n), with a crossreference that A027383 is a(n+1) 2) A077957 Powers of 2 alternating with zeros: 1,0,2,0,4,0,8,0,16,0,32... Action: Add a comment that this is also defined by a(n) = a(n1) + 2*a(n2)  2*a(n3), with a(0)=1, a(1)=0, a(2)=2 3) A056453 Number of palindromes of length n using exactly two different symbols: 0,0,2,2,6,6,14,14,30,30,62,62,126,126,254,254... states as the formulae: a(n) = 2^floor((n+1)/2)  2. a(n) = a(n1) + 2*a(n2)  2*a(n3) Action: No action, as the above entry already appears to cover a(n) = a(n1) + 2*a(n2)  2*a(n3) with a(0)=0, a(1)=0, a(2)=2. 4) (n) = a(n1) + 2*a(n2)  2*a(n3) with a(0)=1, a(1)=0, a(2)=3 produces this interesting looking sequence: 1,0,3,1,7,3,15,7,31,15,63,31,127,63,255,127,511... where every fourth number from every skipped number is the same Action: Suggest this sequence to be added to OEIS 
20180712, 01:48  #4  
Feb 2017
Nowhere
3,011 Posts 
Quote:
x^3  x^2  2*x + 2 = (x  1)*(x^2  2). If m = p, an odd prime, the period (mod p) is twice the multiplicative order of 2 (mod p). 

20180712, 12:11  #5 
Aug 2006
13332_{8} Posts 

20180712, 12:32  #6  
Aug 2006
2×3^{2}×5^{2}×13 Posts 
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Note the link Index entries for linear recurrences with constant coefficients, signature (0,2).which is the same as saying "For large enough n, a(n) = 0*a(n1) + 2*a(n2)" or in this case "For n > 1, a(n) = 2*a(n2)". This is the unique recurrence relation of minimal order (order = number of elements = 2 in this case); you are suggesting signature (1,2,2) which this sequence also satisfies. You can transform that recurrence into yours: a(n) = 2*a(n2) a(n1) = 2*a(n3) a(n1)  2*a(n3) = 0 a(n) = a(n1) + 2*a(n2)  2*a(n3) where the last line is the sum of lines 1 and 3. That does look interesting, go for it. 

20180712, 15:26  #7  
Feb 2017
Nowhere
3,011 Posts 
Quote:
? forstep(i=1,9,[1,2],print(2^i1)) 1 0 3 1 7 3 15 7 31 15 63 31 127 63 255 127 511 255 

20180714, 00:06  #8 
Jul 2018
3_{10} Posts 
So a name for this one could be 'One step back, two steps forward Mersenne numbers'?
With a crossreference to A000225. The sequence modulo m is periodic or eventually periodic with periods being 1 (at n=2^a) or 3 or multiples of 2 (can these periods be ascertained from the (1,0,3) signature, with the '0' pertaining to the evens?): 1,1,4,1,8,4,3,1,12,8,20,4,24,3,8,1,16,12,36,8,12,20,22,4,40,24,36,3,56,8,10,1,20,16,24,12.... 
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