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 A050495 Numbers that are the first term of at least one arithmetic progression with 4 or more terms all having the same value of Euler's totient function phi(x). 9
 72, 144, 216, 216, 288, 432, 432, 576, 648, 648, 792, 864, 864, 1080, 1152, 1224, 1296, 1296, 1368, 1446, 1512, 1584, 1656, 1728, 1728, 1944, 1944, 2088, 2160, 2232, 2304, 2376, 2376, 2448, 2592, 2592, 2664, 2736, 2892, 2952, 3024, 3096, 3168 (list; graph; refs; listen; history; text; internal format)
 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 Cf. A000010, A050496, A050497. Sequence in context: A345794 A157336 A060661 * A137883 A173728 A173547 Adjacent sequences:  A050492 A050493 A050494 * A050496 A050497 A050498 KEYWORD nonn AUTHOR Jud McCranie, Dec 27 1999 STATUS approved

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Last modified September 27 19:23 EDT 2021. Contains 347694 sequences. (Running on oeis4.)