Duplication

[Hans Havermann]

%I A075045
%S A075045 1,3,9,15
%N A075045 Smallest number k such that k and k+1 have n and n+1 
divisors.
%e A075045 a(4) = 15 as 15 has 4 and 16 has 5 divisors. a(6) = 63 as 63 
and 64 have 6 and 7 divisors respectively.

a(6) calculated (from Example) but only 4 terms submitted. At any rate, 
we have:

%I A055927
%S A055927 
1,3,9,15,25,63,121,195,255,361,483,729,841,1443,3363,3481,3721,5041,
%T A055927 
6241,10201,15625,17161,18224,19321,24963,31683,32761,39601,58564,
%U A055927 
59049,65535,73441,88208,110889,121801,143641,145923,149769,167281
%N A055927 Numbers where repetition occurs in A049820.
%C A055927 This means that n+1-d(n+1)=n-d(n), i.e. d(n+1)-d(n)=1, where 
d() is A000005, the number of divisors.