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A054048
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Earliest sequence with a(a(n))=n*(n+1)/2 (Triangular numbers).
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2
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0, 1, 4, 10, 3, 7, 55, 15, 9, 36, 6, 12, 66, 14, 91, 28, 17, 136, 19, 171, 22, 1540, 210, 24, 276, 26, 325, 29, 120, 378, 31, 465, 33, 528, 35, 595, 45, 38, 703, 40, 780, 42, 861, 44, 946, 666, 47, 1081, 49, 1176, 51, 1275, 53, 1378, 56, 21, 1485, 58, 1653
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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LINKS
| Index entries for sequences of the a(a(n)) = 2n family
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EXAMPLE
| a(a(3))=a(10)=6=3*4/2
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MATHEMATICA
| nmax = 58; amax = 2000; tri[n_] := n*(n+1)/2; triQ[n_] := triQ[n] = Resolve[ Exists[ k, Reduce[ k > 0 && n == tri[k], k, Integers] ] ]; t = {{0, a[0] = 0}, {1, a[1] = 1}, {2, a[2]}}; While[ !FreeQ[t, a], t = Table[ {n, a[n]}, {n, 0, nmax}]; n = Select[t, !IntegerQ[#[[2]]]& , 1][[1, 1]]; t2 = Union[ Flatten[ Append[ Select[ t, IntegerQ[ #[[2]] ] & ], n] ] ]; an = If[ n == 2, 4, Select[ Complement[ Range[ Max[t2] ], t2], !triQ[#]&, 1][[1]]]; a[n] = an; While[ an < amax, an = a[n = an] = tri[n] ] ]; A054048 = Table[ a[n], {n, 0, nmax}] (* From Jean-François Alcover, Jan 05 2012 *)
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CROSSREFS
| Cf. A000217.
Sequence in context: A087212 A048870 A070261 * A121512 A010715 A151707
Adjacent sequences: A054045 A054046 A054047 * A054049 A054050 A054051
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KEYWORD
| base,nice,nonn
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AUTHOR
| Henry Bottomley (se16(AT)btinternet.com), Apr 28 2000
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