A077206 - OEIS

A077206

a(1) = 1 and then smallest n-digit triangular number starting with the least significant digit of the previous term and not a multiple of 10.

1, 15, 528, 8001, 10011, 100128, 8006001, 10001628, 800060001, 1000006281, 10000020331, 100000404505, 5000002655781, 10000002437316, 600000012076636, 6000000109431328, 80000000600000001, 100000000447232406

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OFFSET

1,2

LINKS

Table of n, a(n) for n=1..18.

EXAMPLE

a(2) = 15 hence a(3) = 528 is the smallest 3-digit triangular number starting with 5, the least significant digit of 15.

MATHEMATICA

a = {1}, Do[k = Ceiling[(Sqrt[8 # + 1] - 1)/2] &[Last@ IntegerDigits[ a[[i]]] * 10^i]; While[Mod[Set[m, k (k + 1)/2], 10] == 0, k++]; AppendTo[a, m], {i, 17}]; a (* Michael De Vlieger, Jun 11 2017 *)

PROG

(PARI)

{rT(m) = ceil((sqrt(8*m+1)-1)/2)}

{T(n) = (n*(n+1))/2}

my(m=1); for(n=1, 30, my(k=rT((m%10)*10^(n-1))); while(T(k)%10==0, k++); m=T(k); print1(m, ", ")) \\ Max Alekseyev, Feb 15 2005

CROSSREFS

Cf. A000217, A077207.

Sequence in context: A157166 A074040 A002403 * A171197 A351183 A061174

Adjacent sequences: A077203 A077204 A077205 * A077207 A077208 A077209

KEYWORD

base,nonn

AUTHOR

Amarnath Murthy, Nov 02 2002

EXTENSIONS

Corrected and extended by Max Alekseyev, Feb 15 2005

STATUS

approved