A108939 - OEIS

%I #19 Sep 22 2023 02:12:14

%S 2,2,3,2,2,3,5,2,2,3,7,2,2,3,5,2,2,3,11,2,2,3,5,7,13,2,2,3,2,2,3,5,17,

%T 2,2,3,7,19,2,2,3,5,11,2,2,3,23,2,2,3,5,7,13,2,2,3,2,2,3,5,29,2,2,3,7,

%U 11,31,2,2,3,5,17,2,2,3,2,2,3,5,7,13,19,37,2,2,3,2,2,3,5,11,41,2

%N Triangle read by rows in which row n lists all primes p such that p-1|n.

%C Row 2n-1 contains only the term 2.

%H Robert Israel, <a href="/A108939/b108939.txt">Table of n, a(n) for n = 1..10000</a> (rows 1 to 3261, flattened)

%e Row n = 1 : 2 because 1|1.

%e Row n = 2 : 2, 3 because 1|2 and 2|2.

%e Row n = 3 : 2 because 1|3.

%e Row n = 4 : 2, 3, 5 because 1|4, 2|4 and 4|4.

%e Row n = 5 : 2 because 1|5.

%e Row n = 6 : 2, 3, 7 because 1|6, 2|6 and 6|6.

%e Row n = 7 : 2 because 1|7.

%e Row n = 8 : 2, 3, 5 because 1|8, 2|8 and 4|8.

%e Row n = 9 : 2 because 1|9.

%e Row n = 10 : 2, 3, 11 because 1|10, 2|10 and 10|10.

%e Row n = 11 : 2 because 1|11.

%e Row n = 12 : 2, 3, 5, 7, 13 because 1|12, 2|12, 4|12, 6|12 = and 12|12.

%p with(numtheory): for n from 1 to 20 do div:=divisors(n): A:=[seq(div[j]+1,j=1..tau(n))]: B:={}: for i from 1 to tau(n) do if isprime(A[i])=true then B:=B union {A[i]} else B:=B: fi: od: C:=convert(B,list): b[n]:=C: od: for n from 1 to 20 do b[n]:=b[n] od; # yields sequence in triangular form - _Emeric Deutsch_, Aug 03 2005

%Y Row products are A027760. Row sums are A085020. Cf. A067513, A108077.

%K easy,nonn,tabf

%O 1,1

%A _Philippe Deléham_, Jul 20 2005

%E Corrected by _Robert Israel_, Sep 21 2023