THE SMARANDACHE PERIODICAL SEQUENCES by student M.R. Popov, Chandler College 1) Let N be a positive integer with not all digits the same, and N' its digital reverse. Then, let N1 = abs (N-N'), and N1' its digital reverse. Again, let N2 = abs (N1-N1'), N2' its digital reverse, and so on. After a finite number of steps one finds an Nj which is equal to a previous Ni, therefore the sequence is periodical [because if N has, say, n digits, all other integers following it will have n digits or less, hence their number is limited, and one applies the Dirichlet's box principle]. For examples: a) If one starts with N = 27, then N' = 72; abs (27-72) = 45; its reverse is 54; abs (45-54) = 09, ... thus one gets: 27, 45, 09, 81, 63, 27, 45, ... ; the Lenth of the Period LP = 5 numbers (27, 45, 09, 81, 63), and the Lenth of the Sequence 'till the first repetition occurs LS = 5 numbers either. b) If one starts with 52, then one gets: 52, 27, 45, 09, 81, 63, 27, 45, ...; then LP = 5 numbers, while LS = 6. c) If one starts with 42, then one gets: 42, 18, 63, 27, 45, 09, 81, 63, 27, ...; then LP = 5 numbers, while LS = 7. For the sequences of integers of two digits, it seems like: LP = 5 numbers (27, 45, 09, 81, 63; or a circular permutation of them), and 5 <= LS <= 7. Question 1: Find the Lenth of the Period (with its corresponding numbers), and the Lenth of the Sequence 'till the first repetition occurs for: the integers of three digits, and the integers of four digits. (It's easier to write a computer program in these cases to check the LP and LS.) An example for three digits: 321, 198, 693, 297, 495, 099, 891, 693, ...; (similar to the previous period, just inserting 9 in the middle of each number). Generalization for sequences of numbers of n digits. 2) Let N be a positive integer, and N' its digital reverse. For a given positive integer c, let N1 = abs (N'-c), and N1' its digital reverse. Again, let N2 = abs (N1'-c), N2' its digital reverse, and so on. After a finite number of steps one finds an Nj which is equal to a previous Ni, therefore the sequence is periodical [same proof]. For eaxmple: If N = 52, and c =1, then one gets: 52, 24, 41, 13, 30, 02, 19, 90, 08, 79, 96, 68, 85, 57, 74, 46, 63, 35, 52, ...; thus LP = 18, LS = 18. Question 2: Find the Lenth of the Period (with its corresponding numbers), and the Lenth of the Sequence 'till the first repetition occurs (with a given non-null c) for: the integers of two digits, and the integers of three digits. (It's easier to write a computer program in these cases to check the LP and LS.) Generalization for sequences of numbers of n digits. 3) Let N be a positive integer with n digits a1a2...an, and c a given integer > 1. Multiply each digit ai of N by c, and replace ai with the last digit of the product ai x c, say it is bi. Note N1 = b1b2...bn, do the same procedure for N1, and so on. After a finite number of steps one finds an Nj which is equal to a previous Ni, therefore the sequence is periodical [same proof]. For eaxmple: If N = 68 and c = 7: 68, 26, 42, 84, 68, ...; thus LP = 4, LS = 4. Question 3: Find the Lenth of the Period (with its corresponding numbers), and the Lenth of the Sequence 'till the first repetition occurs (with a given c) for: the integers of two digits, and the integers of three digits. (It's easier to write a computer program in these cases to check the LP and LS. Generalization for sequences of numbers of n digits. 4.1) Smarandache generalized periodical sequence: Let N be a positive integer with n digits a1a2...an. If f is a function defined on the set of integers with n digits or less, and the values of f are also in the same set, then: there exist two natural numbers i < j such that f(f(...f(s)...)) = f(f(f(...f(s)...))), where f occurs i times in the left side, and j times in the right side of the previous equality. Particularizing f, one obtains many periodical sequences. Say: If N has two digits a1a2, then: add 'em (if the sum is greater than 10, add the resulted digits again), and substract 'em (take the absolute value) -- they will be the first, and second digit respectively of N1. And same procedure for N1. Example: 75, 32, 51, 64, 12, 31, 42, 62, 84, 34, 71, 86, 52, 73, 14, 53, 82, 16, 75, ... 4.2) More General: Let S be a finite set, and f : S ---> S a function. Then: for any element s belonging to S, there exist two natural numbers i < j such that f(f(...f(s)...)) = f(f(f(...f(s)...))), where f occurs i times in the left side, and j times in the right side of the previous equality. Reference: F. Smarandache, "Sequences of Numbers", University of Craiova Simposium of Students, December 1975.