OFFSET
1,1
COMMENTS
A number can occur multiple times, corresponding to the number of these arithmetic progressions that it starts. - Robert Israel, Nov 29 2016
LINKS
Robert Israel, Table of n, a(n) for n = 1..2704
EXAMPLE
phi(72) = phi(78) = phi(84) = phi(90) = 24, so 72 is a member of the sequence.
MAPLE
N:= 5000: # to get all terms <= N
AP4:= proc(S) local res, n, i1, i4;
n:= nops(S); res:= NULL;
for i1 from 1 to n-3 do
for i4 from i1+3 to n do
if (S[i1] - S[i4]) mod 3 = 0 and has(S, (2*S[i1]+S[i4])/3) and has(S, (S[i1]+2*S[i4])/3) then res:= res, S[i1]
fi
od od;
[res]
end proc:
Res:= NULL:
for m from 1 to N-1 do
Res:= Res, op(select(`<=`, AP4(numtheory:-invphi(m)), N));
od:
sort([Res]); # Robert Israel, Nov 29 2016
CROSSREFS
KEYWORD
nonn
AUTHOR
Jud McCranie, Dec 27 1999
STATUS
approved