cp's OEIS Frontend

A236971 Number of partitions of n into at least 4 parts from which we can form every partition of n into 4 parts by summing elements.

%I A236971 #25 Oct 22 2019 17:34:13
%S A236971 0,0,0,1,2,2,3,3,6,7,8,11,19,21,26,31,52,66,76,88,134,169,215,251,358,
%T A236971 412,517,639,899,1065,1242,1496,2072,2482,2930,3449,4677,5566
%N A236971 Number of partitions of n into at least 4 parts from which we can form every partition of n into 4 parts by summing elements.
%C A236971 The corresponding partitions with 2 in the definition instead of 3 are the complete partitions, which A126796 counts.
%C A236971 The qualifier 'into at least 4 parts' is only relevant for n = 1, 2 or 3. It is included because otherwise the condition would be vacuously true for all partitions of 1, 2 and 3. It seems neater to consider that there are no partitions of 1, 2 or 3 of this form.
%C A236971 What is the limit for large n of the proportion of partitions of n for which this holds, or this sequence divided by A000041?
%e A236971 The valid partitions of 7 are (2, 2, 1, 1, 1), (2, 1, 1, 1, 1, 1) and (1, 1, 1, 1, 1, 1, 1). Given any partition of 7 into 4 parts, we can express these four parts as disjoint sums of elements from these partitions. For the third one this is trivial, for the second one because one element of the partition must be at least 2, for the third because in fact two elements of the partition must be at least 2. So a(7) = 3.
%Y A236971 A000041 counts partitions, A126796 counts complete partitions - the case for partitions into 2 instead of 4, A236970 and A236972 are the cases for 3 and 5 respectively.
%K A236971 nonn,more
%O A236971 1,5
%A A236971 _Jack W Grahl_, Feb 02 2014
%E A236971 a(30)-a(38) from _Willy Van den Driessche_, Oct 22 2019