A391043 Triangle read by rows: T(n,k) is the number of i <= n such that A020481(i) = prime(k).

1, 1, 1, 1, 2, 1, 3, 1, 3, 1, 1, 4, 1, 1, 5, 1, 1, 5, 2, 1, 6, 2, 1, 7, 2, 1, 7, 3, 1, 8, 3, 1, 8, 4, 1, 8, 4, 1, 1, 9, 4, 1, 1, 10, 4, 1, 1, 10, 5, 1, 1, 10, 5, 2, 1, 11, 5, 2, 1, 11, 6, 2, 1, 12, 6, 2, 1, 13, 6, 2, 1, 13, 7, 2, 1, 14, 7, 2, 1, 14, 8, 2, 1, 14, 8, 3, 1, 15, 8, 3, 1, 15, 9, 3

Offset

2,5

Links

Table of n, a(n) for n=2..95.

Example

The triangle T(n, k) begins:
n\k   A020481  1   2   3   4   5   6   7 ...
   2:    2     1
   3:    3     1   1
   4:    3     1   2
   5:    3     1   3
   6:    5     1   3   1
   7:    3     1   4   1
   8:    3     1   5   1
   9:    5     1   5   2
  10:    3     1   6   2
  11:    3     1   7   2
  12:    5     1   7   3
  13:    3     1   8   3
  14:    5     1   8   4
  15:    7     1   8   4   1
  ...

Prog

(PARI) gp(k) = my(j=0); forprime(p=2, k, j++; if (isprime(2*k-p), return(j)));
row(n) = Vecrev(sum(k=2, n, x^gp(k))/x)
for (n=2, 15, print(row(n))) \\ Michel Marcus, Nov 27 2025

Crossrefs

Row sums are A001477(n-1).

Cf. A020481, A377758.

Sequence in context: A046924 A015710 A108415 * A278572 A136644 A111963

Adjacent sequences: A391040 A391041 A391042 * A391044 A391045 A391046

Keyword

nonn,tabf

Author

Michel Eduardo Beleza Yamagishi, Nov 26 2025

Status

approved