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 A237042 UPC check digits. 0
 7, 4, 1, 8, 5, 2, 9, 6, 3, 9, 6, 3, 0, 7, 4, 1, 8, 5, 2, 8, 5, 2, 9, 6, 3, 0, 7, 4, 1, 7, 4, 1, 8, 5, 2, 9, 6, 3, 0, 6, 3, 0, 7, 4, 1, 8, 5, 2, 9, 5, 2, 9, 6, 3, 0, 7, 4, 1, 8, 4, 1, 8, 5, 2, 9, 6, 3, 0, 7, 3, 0, 7, 4, 1, 8, 5, 2, 9, 6, 2, 9, 6, 3, 0, 7, 4, 1, 8, 5, 1, 8, 5, 2, 9, 6, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The UPC check digit is calculated much like a base 10 digital root, except that the digits in the ones place, hundreds place, ten thousands place, etc., are multiplied by 3, before the final step of multiplying the whole sum by -1 prior to taking it modulo 10. Thus the UPC check digit gives the impression of advancing in steps of 7 with each increment of 1 except most of the times when the last digit of n goes from 9 to 0. REFERENCES David Salomon, Coding for Data and Computer Communications. New York: Springer (2006): 41 - 42. LINKS GS1, Check Digit Calculator. Eric W. Weisstein, MathWorld: UPC FORMULA a(n) = -((sum(i = 1 .. floor(L/2), d(2i - 1)) + 3(sum(j = 0 .. floor(L/2), d(2j)))) mod 10, where L is how many digits n has, d(L - 1) is the most significant digit of n, ..., and d(0) is the ones place digit. EXAMPLE a(13) = 0 because 1 * 1 + 3 * 3 = 10, giving a check digit of 0. a(14) = 7 because 1 * 1 + 4 * 3 = 13, and -13 = 7 mod 10. a(15) = 4 because 1 * 1 + 5 * 3 = 16, and -16 = 4 mod 10. CROSSREFS Cf. A231963, A144468. Sequence in context: A010508 A070403 A144468 * A258225 A059630 A258330 Adjacent sequences:  A237039 A237040 A237041 * A237043 A237044 A237045 KEYWORD nonn,easy,base AUTHOR Alonso del Arte, Feb 02 2014 STATUS approved

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Last modified July 20 12:33 EDT 2019. Contains 325180 sequences. (Running on oeis4.)