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 A212697 a(n) = 2*n*3^(n-1). 10
 2, 12, 54, 216, 810, 2916, 10206, 34992, 118098, 393660, 1299078, 4251528, 13817466, 44641044, 143489070, 459165024, 1463588514, 4649045868, 14721978582, 46490458680, 146444944842, 460255540932, 1443528742014, 4518872583696, 14121476824050, 44059007691036 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Main transitions in systems of n particles with spin 1. Consider the set S of all b^n numbers which have n digits in base b. Define as "main transition" a pair (x,y) of elements of S such that x and y differ in base b in only one digit which in y exceeds that in x by 1. This particular sequence a(n) gives the number of such transitions for the case b=3. The terminology originates from quantum theory of coupled spin systems (such as in magnetic resonance) with n particles, each with spin S = (b-1)/2. Then the i-th digit's value in base b can be intended as a label for the b = 2S+1 quantum states of the i-th particle. The most intense main quantum transitions then correspond to the above definition. Due to continuity, the correspondence holds regardless of how strongly coupled are the particles among themselves. a(n) is the number of functions from {1,2,...,n} into {1,2,3} with a specially designated element of the domain that is restricted to be mapped into {1,2}. Hence the e.g.f. is 2*x*exp(x)^3. - Geoffrey Critzer, Mar 01 2015 REFERENCES M. H. Levitt, Spin Dynamics, Basics of Nuclear Magnetic Resonance, 2nd Edition, John Wiley & Sons, 2007, Section 6 (Mathematical techniques). J. A. Pople, W. G. Schneider, H. J. Bernstein, High-Resolution Nuclear Magnetic Resonance, McGraw-Hill, 1959, Chapter 6. LINKS Stanislav Sykora, Table of n, a(n) for n = 1..100 Franck Ramaharo, Statistics on some classes of knot shadows, arXiv:1802.07701 [math.CO], 2018. Index entries for linear recurrences with constant coefficients, signature (6,-9). FORMULA a(n) = n*(b-1)*b^(n-1). For this sequence, set b=3. G.f.: 2*x / (3*x-1)^2; a(n) = 2*A027471(n+1). - R. J. Mathar, Oct 15 2013 a(n) = A005843(n)*A000244(n-1). - Omar E. Pol, Jan 21 2014 a(n) = Sum_{i=1..n} binomial(n,i)*i*2^i. - Geoffrey Critzer, Mar 01 2015 EXAMPLE n=2, b=3, S={00, 01, 02, 10, 11, 12, 20, 21, 22}, main transitions = {(00,01), (00,10), (01,02), (01,12), (02,12), (10,11), (10,20), (11,12), (11,21), (12,22), (20,21), (21,22)}, main transitions count = 12. MAPLE A212697:=n->2*n*3^(n-1): seq(A212697(n), n=1..30); # Wesley Ivan Hurt, Mar 01 2015 MATHEMATICA Table[Sum[Binomial[n, i] i 2^i, {i, n}], {n, 30}] (* Geoffrey Critzer, Mar 01 2015 *) PROG (PARI): mtrans(n, b) = n*(b-1)*b^(n-1); for (n=1, 100, write("b212697.txt", n, " ", mtrans(n, 3))) CROSSREFS Cf. A001787 (b = 2). Cf. A212698, A212699, A212700, A212701, A212702, A212703, A212704 (b = 4, 5, 6, 7, 8, 9, 10). Row n=3 of A258997. Sequence in context: A055703 A242513 A006738 * A111642 A145766 A181765 Adjacent sequences:  A212694 A212695 A212696 * A212698 A212699 A212700 KEYWORD nonn,easy AUTHOR Stanislav Sykora, May 24 2012 STATUS approved

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Last modified July 20 21:42 EDT 2018. Contains 312817 sequences. (Running on oeis4.)