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A077694 a(n) = triangular number pertaining to the number f(n) obtained by concatenating first n natural numbers. 1

%I

%S 1,78,7626,761995,76205685,7620753696,762078456028,76207888812681,

%T 7620789436823655,76207893880582233505,762078938758291471246566,

%U 7620789387578285082874368828,76207893875782400211463469221291

%N a(n) = triangular number pertaining to the number f(n) obtained by concatenating first n natural numbers.

%e a(4) =761995= 1234*1235/2

%t tno[lst_]:=Module[{n=FromDigits[lst]},(n(n+1))/2]; tno/@Table[Flatten[ IntegerDigits/@Range[n]],{n,20}] (* _Harvey P. Dale_, Aug 16 2011 *)

%Y Cf. A077692, A077693.

%K base,nonn

%O 1,2

%A _Amarnath Murthy_, Nov 18 2002

%E More terms from _Sascha Kurz_, Jan 07 2003

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Last modified February 2 06:25 EST 2023. Contains 360000 sequences. (Running on oeis4.)