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 A003278 Szekeres's sequence: a(n)-1 in ternary = n-1 in binary; also: a(1) = 1, a(2) = 2, a(n) is smallest number k which avoids any 3-term arithmetic progression in a(1), a(2), ..., a(n-1), k. (Formerly M0975) 61
 1, 2, 4, 5, 10, 11, 13, 14, 28, 29, 31, 32, 37, 38, 40, 41, 82, 83, 85, 86, 91, 92, 94, 95, 109, 110, 112, 113, 118, 119, 121, 122, 244, 245, 247, 248, 253, 254, 256, 257, 271, 272, 274, 275, 280, 281, 283, 284, 325, 326, 328, 329, 334, 335, 337, 338, 352, 353 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS That is, there are no three elements A, B and C such that B - A = C - B. Difference sequence related to Gray code bit sequence (A001511). The difference patterns follows a similar repeating pattern (ABACABADABACABAE...), but each new value is the sum of the previous values, rather than simply 1 more than the maximum of the previous values. - Hal Burch (hburch(AT)cs.cmu.edu), Jan 12 2004 Sums of distinct powers of 3, translated by 1. Positions of 0 in A189820; complement of A189822. - Clark Kimberling, May 26 2011 Also, Stanley sequence S(1): see OEIS index to Stanley sequences. - M. F. Hasler, Jan 18 2016 REFERENCES S. R. Finch, Mathematical Constants, Cambridge, 2003, p. 164. R. K. Guy, Unsolved Problems in Number Theory, E10. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS David W. Wilson, Table of n, a(n) for n = 1..10000  a(1..1024) from T. D. Noe J.-P. Allouche and J. Shallit, The ring of k-regular sequences, Theoretical Computer Sci., 98 (1992), 163-197. J.-P. Allouche and J. Shallit, The ring of k-regular sequences, Theoretical Computer Sci., 98 (1992), 163-197. P. ErdÅ‘s and P. Turan, On some sequences of integers, J. London Math. Soc., 11 (1936), 261-264. Joseph Gerver, James Propp and Jamie Simpson, Greedily partitioning the natural numbers into sets free of arithmetic progressions Proc. Amer. Math. Soc. 102 (1988), no. 3, 765-772. F. Iacobescu, Smarandache Partition Type and Other Sequences, Bull. Pure Appl. Sci. 16E, 237-240, 1997. H. Ibstedt, A Few Smarandache Sequences, Smarandache Notions Journal, Vol. 8, No. 1-2-3, 1997, 170-183. Gabor Korvin, Short note: Every large set of integers contains a three term arithmetic progression arXiv 1404.1557 [math.NT], Apr 6 2014. James Propp and N. J. A. Sloane, Email, March 1994 F. Smarandache, Sequences of Numbers Involved in Unsolved Problems. R. P. Stanley, Letter to N. J. A. Sloane, c. 1991 Eric Weisstein's World of Mathematics, Smarandache Sequences FORMULA a(2k + 2) = a(2k + 1) + 1, a(2^k + 1) = 2*a(2^k). a(n) = b(n+1) with b(0)=1, b(2n)=3b(n)-2, b(2n+1)=3b(n)-1. - Ralf Stephan, Aug 23 2003 G.f.: x/(1-x)^2 + x * sum(3^(k-1)*x^(2^k)/((1-x^(2^k))*(1-x)),k=1..infinity). - Ralf Stephan, Sep 10 2003, corrected by Robert Israel, May 25 2011 Conjecture: a(n) = (A191107(n) + 2)/3 = (A055246(n) + 5)/6. - L. Edson Jeffery, Nov 26 2015 EXAMPLE G.f. = x + 2*x^2 + 4*x^3 + 5*x^4 + 10*x^5 + 11*x^6 + 13*x^7 + 14*x^8 + 28*x^9 + ... MAPLE a:= proc(n) local m, r, b; m, r, b:= n-1, 1, 1;       while m>0 do r:= r+b*irem(m, 2, 'm'); b:= b*3 od; r     end: seq(a(n), n=1..100); # Alois P. Heinz, Aug 17 2013 MATHEMATICA Take[ Sort[ Plus @@@ Subsets[ Table[3^n, {n, 0, 6}]]] + 1, 58] (* Robert G. Wilson v, Oct 23 2004 *) a[1] = 0; h = 180; Table[a[3 k - 2] = a[k], {k, 1, h}]; Table[a[3 k - 1] = a[k], {k, 1, h}]; Table[a[3 k] = 1, {k, 1, h}]; Table[a[n], {n, 1, h}]   (* A189820 *) Flatten[Position[%, 0]]  (* A003278 *) Flatten[Position[%%, 1]] (* A189822 *) (* A003278 from A189820, from Clark Kimberling, May 26 2011 *) Table[FromDigits[IntegerDigits[n, 2], 3] + 1, {n, 0, 57}] (* Amit Munje, Jun 03 2018 *) PROG (Perl) \$nxt = 1; @list = (); for (\$cnt = 0; \$cnt < 1500; \$cnt++) { while (exists \$legal{\$nxt}) { \$nxt++; } print "\$nxt "; last if (\$nxt >= 1000000); for (\$i = 0; \$i <= \$#list; \$i++) { \$t = 2*\$nxt - \$list[\$i]; \$legal{\$t} = -1; } \$cnt++; push @list, \$nxt; \$nxt++; } # Hal Burch (PARI) a(n)=1+sum(i=1, n-1, (1+3^valuation(i, 2))/2) \\ Ralf Stephan, Jan 21 2014 (Python) def A003278(n): ....return int(format(n-1, 'b'), 3)+1 # Chai Wah Wu, Jan 04 2015 CROSSREFS Equals 1 + A005836. Cf. A001511, A098871. Row 0 of array in A093682. Summary of increasing sequences avoiding arithmetic progressions of specified lengths (the second of each pair is obtained by adding 1 to the first): 3-term AP: A005836 (>=0), A003278 (>0); 4-term AP: A005839 (>=0), A005837 (>0); 5-term AP: A020654 (>=0), A020655 (>0); 6-term AP: A020656 (>=0), A005838 (>0); 7-term AP: A020657 (>=0), A020658 (>0); 8-term AP: A020659 (>=0), A020660 (>0); 9-term AP: A020661 (>=0), A020662 (>0); 10-term AP: A020663 (>=0), A020664 (>0). See also A003002. Sequence in context: A220696 A275482 A156799 * A236246 A004792 A167795 Adjacent sequences:  A003275 A003276 A003277 * A003279 A003280 A003281 KEYWORD nonn,nice,easy AUTHOR STATUS approved

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Last modified August 21 06:23 EDT 2018. Contains 313934 sequences. (Running on oeis4.)