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A284153
Pairs of integers (x, y) such that D(x+y+1) = D(x) union D(y), 1 < y < x, where D(n) is the set of distinct prime divisors of n.
1
3, 2, 5, 4, 8, 3, 9, 2, 9, 5, 9, 8, 17, 16, 20, 9, 24, 5, 27, 8, 27, 14, 32, 3, 32, 11, 33, 32, 49, 20, 50, 9, 54, 5, 54, 11, 56, 27, 65, 64, 80, 9, 81, 41, 98, 27, 99, 32, 104, 25, 125, 9, 125, 14, 125, 24, 125, 49, 125, 54, 125, 84, 128, 43, 129, 128, 144, 5
OFFSET
1,1
COMMENTS
The pairs of the form (2^k+1, 2^k) are members => the sequence is infinite.
LINKS
EXAMPLE
The pair (20, 9) is in the sequence because D(20) = {2, 5}, D(9) = {3} => D(20 + 9 + 1) = D(30) = {2, 3, 5} = D(20) union D(9).
MAPLE
with(numtheory):nn:=150:
for a from 3 to nn do:
x:=factorset(a):n0:=nops(x):A:={op(x), x[n0]}:
for b from 2 to a-1 do:
y:=factorset(b):n1:=nops(y):B:={op(y), y[n1]}:
z:=factorset(a+b+1):n2:=nops(z):C:={op(z), z[n2]}:
if C = A union B
then
printf(`%d, `, a):printf(`%d, `, b):else
fi:
od:
od:
MATHEMATICA
d[n_] := First /@ FactorInteger[n]; Flatten@ Reap[ Do[ dx = d[x]; Do[ If[ d[x + y + 1] == Union[dx, d[y]], Sow[{x, y}]], {y, x-1}], {x, 2, 144}]][[2, 1]] (* Giovanni Resta, Mar 23 2017 *)
PROG
(Python)
from sympy.ntheory import primefactors
l=[]
for x in range(2, 145):
for y in range(2, x):
if primefactors(x + y + 1) == sorted(primefactors(x) + primefactors(y)):
l+=[x, y]
print(l) # Indranil Ghosh, Mar 21 2017
CROSSREFS
Sequence in context: A137707 A164380 A143527 * A266794 A340286 A194078
KEYWORD
nonn
AUTHOR
Michel Lagneau, Mar 21 2017
STATUS
approved