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 A261696 a(n) is least number > 0 such that the concatenation of a(1) ... a(n) is 17-gonal: (15n^2 - 13n)/2. 6
 1, 7, 689, 6797, 67984832, 6798483348333332, 8455610150480042707742277762479, 707328322040172689545426423113211907561874137758547957769721082 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS From Chai Wah Wu, Mar 16 2018: (Start) There are some interesting patterns observed in the terms. Terms a(5), a(6), a(9), a(10), a(11), a(12), ... share the same prefix of 6798483... From terms a(n) for n > 5, there seems to a pattern of how they are constructed from previous terms. a(6) is formed by inserting 3483...3 between the penultimate digit and the last digit of a(5). Then a(7) and (8) do not follow this pattern. The digits of a(9) and a(6) match until the last digit of a(6). Next, a(10), a(11) and (12) are formed from a(9), a(10) and a(11) resp. by inserting 3483...3. Then this pattern is interrupted by a(13) and a(14), and continue again for a(15) ..., etc. (End) LINKS Chai Wah Wu, Table of n, a(n) for n = 1..11 Wikipedia, Polygonal number Chai Wah Wu, terms < 10^70000 to illustrate the pattern observed (see comments) EXAMPLE 1, 17, 17689, 176896797 are 17-gonal. PROG (PARI) heptadecagonal(n)=ispolygonal(n, 17) first(m)=my(s=""); s="1"; print1(1, ", "); for(i=2, m, n=1; while(!heptadecagonal(eval(concat(s, Str(n)))), n++); print1(n, ", "); s=concat(s, Str(n))) CROSSREFS Cf. A051671, A051869 (17-gonal numbers), A061109, A061110, A264733, A264738, A264776, A264777, A264842, A264848, A264849, A264804. Sequence in context: A038803 A144957 A163016 * A174855 A186160 A129291 Adjacent sequences: A261693 A261694 A261695 * A261697 A261698 A261699 KEYWORD nonn,base AUTHOR Anders Hellström, Nov 26 2015 EXTENSIONS a(6)-a(8) from Chai Wah Wu, Mar 16 2018 STATUS approved

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Last modified September 29 18:47 EDT 2023. Contains 365775 sequences. (Running on oeis4.)