A061352 First row of array shown below.

1, 2, 6, 7, 5, 6, 8, 9, 5, 6, 6, 7, 1, 2, 0, 1, 3, 4, 0, 1, 1, 2, 6, 7, 5, 6, 8, 9, 5, 6, 6, 7, 1, 2, 0, 1, 3, 4, 0, 1, 1, 2, 6, 7, 5, 6, 8, 9, 5, 6, 6, 7, 1, 2, 0, 1, 3, 4, 0, 1, 1, 2, 6, 7, 5, 6, 8, 9, 5, 6, 6, 7, 1, 2, 0, 1, 3, 4, 0, 1, 1, 2, 6, 7, 5, 6, 8, 9, 5, 6, 6, 7, 1, 2, 0, 1, 3, 4, 0, 1, 1, 2, 6, 7, 5

Offset: 1

Author

Amarnath Murthy, Apr 28 2001

Keywords

• nonn
• easy

Comments

Least significant digit of the n-th triangular number if n is odd.

Examples

			1 2 6 7 5 6 8 9 5 ...
3 5 8 4 7 7 0 4 ...
4 9 3 8 6 1 3 ...
0 2 9 5 2 2 ...
1 0 4 3 1 ...
1 3 4 0 ...
		

Crossrefs

Cf. A061353.

Formula

a(1)=1; if n odd, a(n+1)=(a(n)+1) mod 10; if n even, a(n+1)=(a(n)+2n) mod 10.

a(2n-1) = n*(2n-1)- 10*Floor[n*(2n-1)/10], a(2n) = n(2n+1)- 10*Floor[n*(2n+1)/10] + 1.

Extensions

More terms and formula from Larry Reeves (larryr(AT)acm.org), May 15 2001