# [seqfan] two dumb questions

Peter Lawrence peterl95124 at sbcglobal.net
Sat Jul 22 03:43:47 CEST 2017

```A051731: characteristic triangle of divisors of n

A000005: = number of divisors of n   =   row sums of A051731

1.  How is A051731 different from Redheffer matrix A143104,
which seems to have same FORMULA definition ?
a(i,j) = 1 if j=1 or j|i; 0 otherwise.

Axxxxxx:  characteristic triangle of numbers with gcd( I, j ) = 1

A000010 = number of relative-primes of n   =   row sums of Axxxxxx

a(i,j) = 1 if gcd(j,i) = 1; 0 otherwise.

1:   1,                                               1
2:   1, 0,                                            1
3:   1, 1, 0,                                         2
4:   1, 0, 1, 0,                                      2
5:   1, 1, 1, 1, 0,                                   4
6:   1, 0, 0, 1, 0, 0,                                2
7:   1, 1, 1, 1, 1, 1, 0,                             6
8:   1, 0, 1, 0, 1, 0, 1, 0,                          4
9:   1, 1, 0, 1, 1, 0, 1, 1, 0,                       6
10:   1, 0, 1, 0, 0, 0, 1, 0, 1, 0,                    4
11:   1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0,                 10
12:   1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0,              4
13:   1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0,           12
14:   1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 1, 0,        6
15:   1, 1, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 1, 0,     8
16:   1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0,  8

2.    I suspect I computed this triangle incorrectly, because I can’t find it in the OEIS  _?_

Peter Lawrence.
```