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A285885 Ulam numbers n such that 3*n is also an Ulam number. 3
1, 2, 6, 16, 38, 138, 182, 209, 309, 341, 612, 1030, 1389, 2513, 2584, 2628, 2650, 2750, 3031, 3207, 3290, 3593, 3742, 3874, 3962, 4121, 4155, 4998, 5384, 5797, 6552, 6723, 6833, 7461, 7979, 8453, 8541, 8844, 8949, 9015, 9164, 9577, 10547, 10569, 11197, 11346 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
It appears that there are many more values in this sequence than in A068791, Ulam numbers n such that 2*n is also an Ulam number.
LINKS
MAPLE
N:= 40000: # for terms <= N/3
V:= Vector(N):
U:= [1, 2]:
V[3]:= 1:
for i from 3 do
found:= false;
for j from U[i-1]+1 to N do
if V[j]=1 then found:= true; break fi
od;
if not found then break fi;
R:= select(`<=`, j+~U, N):
V[R]:= 1 +~ V[R];
U:= [op(U), j];
od:
U:= convert(U, set):
sort(convert(U intersect map(`*`, U, 1/3), list)); # Robert Israel, Aug 31 2020
CROSSREFS
Cf. A068791.
Sequence in context: A265107 A217631 A046209 * A273348 A198951 A371709
KEYWORD
nonn
AUTHOR
Enrique Navarrete, Apr 27 2017
STATUS
approved

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Last modified May 23 06:14 EDT 2024. Contains 372760 sequences. (Running on oeis4.)