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A222603 a(1)=1; for n>0, a(n+1) is the least practical number q>a(n) such that 2(a(n)+1)-q is practical. 2
1, 2, 4, 6, 8, 12, 18, 20, 24, 30, 32, 36, 42, 54, 56, 60, 66, 78, 80, 84, 90, 104, 120, 162, 176, 192, 210, 224, 234, 260, 270, 272, 276, 294, 320, 330, 342, 378, 380, 384, 390, 392, 396, 414, 416, 420, 450, 462, 464, 468, 476, 486, 510, 512, 522, 546, 594, 620, 630, 702, 704, 714, 726, 728, 744, 750, 798, 800, 810, 812, 816, 920, 924, 930, 966, 968, 972, 980, 990, 992, 1014, 1040, 1050, 1088, 1122, 1232, 1242, 1254, 1280, 1290, 1302, 1316, 1332, 1350, 1352, 1380, 1386, 1458, 1518, 1520 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

By a result of Melfi, each positive even number can be written as the sum of two practical numbers.

For a practical number p, define h(p) as the least practical number q>p such that 2(p+1)-q is practical. Construct a simple (undirected) graph H as follows: The vertex set of H is the set of all practical numbers, and for two vertices p and q>p there is an edge connecting p and q if and only if h(p)=q. Clearly H contains no cycle.

Conjecture: The graph H constructed above is connected and hence it is a tree.

LINKS

Zhi-Wei Sun, Table of n, a(n) for n = 1..10000

G. Melfi, On two conjectures about practical numbers, J. Number Theory 56 (1996) 205-210 [MR96i:11106].

EXAMPLE

a(4)=6 since 2(a(3)+1)=10=6+4 with 4 and 6 both practical, and 6>a(3)=4.

MATHEMATICA

f[n_]:=f[n]=FactorInteger[n]

Pow[n_, i_]:=Pow[n, i]=Part[Part[f[n], i], 1]^(Part[Part[f[n], i], 2])

Con[n_]:=Con[n]=Sum[If[Part[Part[f[n], s+1], 1]<=DivisorSigma[1, Product[Pow[n, i], {i, 1, s}]]+1, 0, 1], {s, 1, Length[f[n]]-1}]

pr[n_]:=pr[n]=n>0&&(n<3||Mod[n, 2]+Con[n]==0)

k=1

n=1

Do[If[m==1, Print[n, " ", 1]]; If[m==k, n=n+1; Do[If[pr[2j]==True&&pr[2m+2-2j]==True, k=2j; Print[n, " ", 2j]; Goto[aa]], {j, Ceiling[(m+1)/2], m}]];

Label[aa]; Continue, {m, 1, 1000}]

CROSSREFS

Cf. A005153, A222532, A163846, A163847, A222566.

Sequence in context: A081954 A096903 A273457 * A177710 A032458 A284005

Adjacent sequences:  A222600 A222601 A222602 * A222604 A222605 A222606

KEYWORD

nonn

AUTHOR

Zhi-Wei Sun, Feb 26 2013

STATUS

approved

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Last modified September 18 05:40 EDT 2021. Contains 347509 sequences. (Running on oeis4.)