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 A306908 Numbers k with exactly three distinct prime factors and such that phi(k) is a square. 4
 60, 114, 126, 170, 204, 240, 273, 285, 315, 364, 370, 380, 438, 444, 456, 468, 504, 540, 680, 816, 825, 902, 960, 969, 978, 1010, 1026, 1071, 1095, 1100, 1134, 1212, 1258, 1292, 1358, 1456, 1460, 1480, 1500, 1520, 1729, 1746, 1752, 1776, 1824, 1836, 1872 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence is the intersection of A033992 and A039770. The integers with only one prime factor and whose totient is a square are in A002496 and A054755, the integers with two prime factors and whose totient is a square are in A324745, A324746 and A324747. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 Bernard Schott, Subfamilies and subsequences FORMULA 1st family: The primitive terms are p*q*r with p,q,r primes and phi(p*q*r) = (p-1)*(q-1)*(r-1) = m^2. These primitives generate the entire family formed by the numbers k = p^(2s+1) * q^(2t+1) * r^(2u+1) with s,t,u >= 0, and phi(k) = (p^s * q^t * r^u * m)^2. 2nd family: The primitive terms are p^2 * q * r with p,q,r primes and phi(p^2 * q * r) = p*(p-1)*(q-1)*(r-1) = m^2. These primitives generate the entire family formed by the numbers k = p^(2s) * q^(2t+1) * r^(2u+1) with s >= 1, t,u >= 0, and phi(k) = (p^(s-1) * q^t * r^u * m)^2. 3rd family: The primitive terms are p^2 * q^2 * r with p,q,r primes and phi(p^2 * q^2 * r) = p*q*(p-1)*(q-1)*(r-1) = m^2. These primitives generate the entire family formed by the numbers k = p^(2s) * q^(2t) * r^(2u+1) with s,t> = 1, u >= 0, and phi(k) = (p^(s-1) * q^(t-1) * r^u * m)^2. EXAMPLE 1st family: 273 = 3 * 7 * 13 and phi(273) = 12^2. 2nd family: 816 = 2^4 * 3 * 17 and phi(816) = 16^2. 3rd family: 6975 = 3^2 * 5^2 * 31 and phi(6975) = 60^2. MAPLE filter:= n -> issqr(numtheory:-phi(n)) and nops (numtheory:-factorset(n))=3: select(filter, [\$1..2000]); # after Robert Israel in A324745 MATHEMATICA Select[Range[2000], And[PrimeNu@ # == 3, IntegerQ@ Sqrt@ EulerPhi@ #] &] (* Michael De Vlieger, Mar 31 2019 *) PROG (PARI) isok(n) = (omega(n)==3) && issquare(eulerphi(n)); \\ Michel Marcus, Mar 19 2019 CROSSREFS Intersection of A033992 and A039770. Cf. A002496, A054755 (only one prime factor), A324745, A324746, A324747 (two prime factors). Sequence in context: A217738 A182683 A174601 * A252961 A252962 A296767 Adjacent sequences:  A306905 A306906 A306907 * A306909 A306910 A306911 KEYWORD nonn AUTHOR Bernard Schott, Mar 16 2019 STATUS approved

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Last modified August 2 18:40 EDT 2021. Contains 346428 sequences. (Running on oeis4.)