OFFSET
1,1
COMMENTS
L(x/y) = L(y/x) for primes x, y, and either x = 4r+1 for some r, or y = 4r+1 for some r.
We observe pairs of the form (a(n), a(n)+1) = (55, 56), (94, 95), (111, 112), (145, 146), (183, 184), (205, 206), (274, 275), (291, 292), (327, 328), ..., .
From Robert G. Wilson v, Nov 16 2016: (Start)
We observe triples of the form (a(n), a(n)+1, a(n)+2) with a(n): 542, 543, 655, 1262, 1411, 1646, 1982, 2305, 2306, 2942, 3025, ..., .
We observe quadruples of the form (a(n), a(n)+1, a(n)+2, a(n)+3) with a(n): 542, 2305, 7022, 10081, 19981, 35821, 41372, 50011, 50941, ..., .
We observe quintuples of the form (a(n), a(n)+1, a(n)+2, a(n)+3, a(n)+4) with a(n): 85631, ..., .
Number of terms less than 10^k: 0, 14, 168, 1591, 14175, 127791, ...
Number of twin terms less than 10^k: 0, 2, 31, 250, 1737, 13604, ...
(End)
LINKS
Robert G. Wilson v, Table of n, a(n) for n = 1..10000
Eric Weisstein's World of Mathematics, Legendre Symbol
EXAMPLE
14 is in the sequence because the prime factors are 2 and 7 => L(2,7)= L(7,2)= 1.
137678 is in the sequence because the prime factors are 2, 23, 41 and 73 => L(2,23) = L(2,41) = L(2,73) = L(23,41) = L(23,73) = L(41,73) = L(73,41) = L(73,23) = L(73,2) = L(41,23) = L(41,2) = L(23,2) = 1.
MAPLE
with(numtheory): nn:=400:
for n from 1 to nn do:
x:=factorset(n):n0:=nops(x):
if n0>1
then
ii:=0:
for i from 1 to n0-1 while(ii=0) do:
for j from 2 to n0 while(ii=0) do:
p:=legendre(x[i], x[j]):q:=legendre(x[j], x[i]):
if p=-1 or q=-1 then ii:=1:
else fi:
od:od:
if ii=0 then printf(`%d, `, n):
else fi:fi:
od:
MATHEMATICA
fQ[n_] := If[ CompositeQ@ n, Block[{pf = Transpose[ FactorInteger[n]][[1]]}, lng = Length@ pf; Union[ Flatten[ Table[ JacobiSymbol[pf[[i]], pf[[j]]], {i, lng}, {j, lng}]]] == {0, 1}], False]; Select[ Range@ 330, fQ] (* Robert G. Wilson v, Nov 16 2016 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Nov 16 2016
STATUS
approved