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

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} m=1; for(n=2, 30, 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. 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