A330381 Triangle read by rows: T(n,k) is the number of ternary strings of length n with k indispensable digits, with 0 <= k <= n.

1, 1, 2, 1, 5, 3, 1, 9, 13, 4, 1, 14, 35, 26, 5, 1, 20, 75, 96, 45, 6, 1, 27, 140, 267, 216, 71, 7, 1, 35, 238, 623, 750, 427, 105, 8, 1, 44, 378, 1288, 2123, 1800, 770, 148, 9, 1, 54, 570, 2436, 5211, 6046, 3858, 1296, 201, 10, 1, 65, 825, 4302, 11505, 17303

Offset: 0

Author

Ji Young Choi, Dec 12 2019

Keywords

• nonn
• base
• tabl

Comments

A digit in a string is called indispensable, if it is greater than the following digit or equal to the following digits which are eventually greater than the following digit. We also assume that there is an invisible digit 0 at the end of any string. For example, in 7233355548, the digits 7, 5, 5, 5, and 8 are indispensable.

T(n, k) is also the number of integers m where the length of the ternary representation of m is less than or equal to n and the digit sum of the ternary representation of 2m is 2k.

Examples

			Triangle begins
  1;
  1,  2;
  1,  5,  3;
  1,  9, 13,  4;
  1, 14, 35, 26,  5;
  1, 20, 75, 96, 45, 6;
  ...
There is 1 string (00) of length 2 with 0 indispensable digits.
There are 5 strings (01, 02, 10, 20, 12) of length 2 with 1 indispensable digit.
There are 3 strings (11, 21, 22) of length 2 with 2 indispensable digits.
Hence T(2, 0) = 1, T(2, 1) = 5, T(2, 2) = 3.
		

Links

• J. Y. Choi, Indispensable digits for digit sums, Notes Number Theory Discrete Math 25 (2019), pp. 40-48.
• J. Y. Choi, Digit sums generalizing binomial coefficients, J. Integer Seq. 22 (2019), Article 19.8.3.

Crossrefs

Cf. A027907, A330509.

Programs

Mathematica

Table[Total@ Map[Sum[Binomial[n, i] Binomial[n - i, # - 2 i], {i, 0, n}] &, 2 k + {-1, 0}], {n, 0, 10}, {k, 0, n}] // Flatten (* Michael De Vlieger, Dec 23 2019, after Adi Dani at A027907 *)

PARI

A027907(n, k) = if(n<0, 0, polcoeff((1 + x + x^2)^n, k));
T(n, k) = A027907(n, 2*k-1) + A027907(n, 2*k); \\ Jinyuan Wang, Dec 14 2019

Formula

T(n, k) = A027907(n, 2k-1) + A027907(n, 2k).

Extensions

More terms from Jinyuan Wang, Dec 14 2019