A077507 Largest n-digit triangular number beginning with n.

1, 28, 378, 4950, 59685, 699153, 7998000, 89987820, 999961560, 1099969156, 11999870821, 129999769851, 1399999074540, 14999999588925, 159999994275696, 1699999973850921, 17999999979109470, 189999999508759300, 1999999999000000000, 20999999994116393253

Offset

1,2

Links

Table of n, a(n) for n=1..20.

Prog

(Python)
from sympy import integer_nthroot
def a(n):
  target = 2 * int(str(n) + '9'*(n-len(str(n))))
  r, exact = integer_nthroot(target, 2)
  while (r-1)*r//2 <= target//2: r += 1
  return (r-2)*(r-1)//2
print([a(n) for n in range(1, 21)]) # Michael S. Branicky, Jan 28 2021

Crossrefs

Cf. A000217, A077506 (smallest).

Sequence in context: A122732 A010944 A022623 * A130608 A331353 A177108

Adjacent sequences: A077504 A077505 A077506 * A077508 A077509 A077510

Keyword

base,nonn

Author

Amarnath Murthy, Nov 08 2002

Extensions

More terms from Sascha Kurz, Jan 04 2003

Status

approved