A077694 - OEIS

A077694

a(n) = triangular number whose index is the concatenation of the first n natural numbers.

1, 78, 7626, 761995, 76205685, 7620753696, 762078456028, 76207888812681, 7620789436823655, 76207893880582233505, 762078938758291471246566, 7620789387578285082874368828, 76207893875782400211463469221291, 762078938757823958287474561318103955

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..14.

FORMULA

a(n) = A000217(A007908(n)). - Sean A. Irvine, Jun 06 2025

EXAMPLE

a(4) = 761995 = 1234*1235/2.

MAPLE

a:= n-> (t-> t*(t+1)/2)(parse(cat($1..n))):

seq(a(n), n=1..15); # Alois P. Heinz, Jun 06 2025

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

Cf. A000217, A007908, A077692, A077693.

Sequence in context: A328354 A268023 A268016 * A119216 A119236 A383942

Adjacent sequences: A077691 A077692 A077693 * A077695 A077696 A077697

KEYWORD

base,nonn

AUTHOR

Amarnath Murthy, Nov 18 2002

EXTENSIONS

More terms from Sascha Kurz, Jan 07 2003

STATUS

approved