OFFSET
1,1
COMMENTS
The first 112 terms are successive multiples of 30, but this sequence does not coincide with A249674: a(113) = 210. See the Khovanova link and comments in A050518.
The increments <= 1000 for terms of A050518 between 13413600 and 10^9 (see comment on A050518) are 720, 750, 780, 810, 840, 870, 900, 930, 960, 990, 210, 210, 420, 30, 420, 630, 630, 840, 60, 840, 90, 30, 30, 120, 150, 30, 18. - Mauro Fiorentini, Apr 12 2015
LINKS
Robert Israel, Table of n, a(n) for n = 1..114
Tanya Khovanova, Non Recursions
MAPLE
N:= 10^7: # to get all terms <= N in A050518
with(numtheory):
Res:= NULL:
phis:= {seq(phi(i), i=2..N)}:
for m in phis do
S:= convert(invphi(m), set);
if nops(S) < 6 then next fi;
for d from 0 to 4 do
Sd[d]:= select(t-> (t mod 5 = d), S, d);
nd:= nops(Sd[d]);
for i0 from 1 to nd-1 do
s0:= Sd[d][i0];
if s0 > N then break fi;
for i5 from i0+1 to nd do
s5:= Sd[d][i5];
incr:= (s5 - s0)/5;
if {s0+incr, s0+2*incr, s0+3*incr, s0+4*incr} subset S then
Res:= Res, [s0, incr];
fi
od
od;
od;
od:
sort([Res], (s, t)->s[1]<t[1]); # gives both A050518 and A050519 entries map2(op, 2, %); # Robert Israel, Apr 30 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Jud McCranie, Dec 28 1999
EXTENSIONS
More terms from Mauro Fiorentini, Apr 12 2015
STATUS
approved