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A077694
a(n) = triangular number pertaining to the number f(n) obtained by concatenating first n natural numbers.
1
1, 78, 7626, 761995, 76205685, 7620753696, 762078456028, 76207888812681, 7620789436823655, 76207893880582233505, 762078938758291471246566, 7620789387578285082874368828, 76207893875782400211463469221291
OFFSET
1,2
EXAMPLE
a(4) =761995= 1234*1235/2
MATHEMATICA
tno[lst_]:=Module[{n=FromDigits[lst]}, (n(n+1))/2]; tno/@Table[Flatten[ IntegerDigits/@Range[n]], {n, 20}] (* Harvey P. Dale, Aug 16 2011 *)
CROSSREFS
Sequence in context: A328354 A268023 A268016 * A119216 A119236 A126998
KEYWORD
base,nonn
AUTHOR
Amarnath Murthy, Nov 18 2002
EXTENSIONS
More terms from Sascha Kurz, Jan 07 2003
STATUS
approved