decompwlj 3D

Numbers k such that k and k-1 are composite

A005381 on the OEIS · family complement

Weight–level plate of Numbers k such that k and k-1 are composite
Click the plate to explore it in 3-D. Weight k across, level L up, both on log scales: blue in the weight class (k ≤ L), orange in the level class (k > L). The dashed diagonal is k = L.
Open in the 3-D viewerA005381 on the OEIS
Terms100,000 (n = 1 … 100,000)
Decomposable (a > 2d)99,999
Level class, k > L2,460 · 2.46 %
Weight class, k ≤ L97,539 · 97.54 %
Ties, k = L88
On the level line L = 11,446
Forced level, l ≤ d²2
Range of a(n)9 … 123,152
Range of the jump d1 … 5
Largest weight k, level L122,953, 61,575

The gaps are 1, 3 and 5; the level share is 2.46 %; L = 1 holds 59 % of the level class.

The decomposition

Every term of a strictly increasing sequence is written a(n) = k(n)·L(n) + d(n): the jump d = a(n+1) − a(n), the weight k the least divisor of a − d greater than d, the level L = (a − d)/k. A term decomposes when a > 2d; it is in the level class when k > L and in the weight class when k ≤ L. See decompwlj.com and arXiv:0711.0865, or how it works, with worked examples and a live one.