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 A261657 Numbers with 3 prime factors a < b < c such that 2c = a^4 + b^2. 3
 795, 3333, 11795, 20515, 25805, 38845, 40107, 48453, 66355, 106285, 108363, 183673, 184445, 236365, 265955, 329063, 347883, 605635, 856595, 1005715, 1068267, 1307047, 1356035, 1901485, 1955787, 2469379, 2733565, 3229795, 3571867 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence is interesting as a less constrained form of A261656, or as a 3-prime-factored form of A261658. Unlike A261656, this sequence has many examples of the b and c of one member being the a and b of another. Two such members of this sequence are 3333=3*11*101 and 13799731 = 11*101*12421. This lets us consider a four factor composite number using both, giving 3*11*101*12421 = 41399193. The sequence of four-factor composites such as this is A261658. It would be nice to know if, in general, analogous n-factor composites exist. LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 EXAMPLE The prime factors of 795 are 3, 5, and 53. 3^4=81, 5^2=25, and the average of 81 and 25 gives 53. Thus, 795 is a member. The prime factors of 3333 are 3, 11, and 101. 3^4=81, 11^2=121, and the average of 81 and 121 gives 101. Thus, 3333 is a member. MAPLE n := 20: L := []: for a from 3 to n do if isprime(a) then for b from a to n^2 do if isprime(b) then c := (a^4+b^2)*(1/2); if isprime(c) then L := [op(L), a*b*c] end if end if end do end if end do; L := sort(L): L := remove(proc (t) options operator, arrow; (3/2)*n^2*(n^4+9) < t end proc, L) PROG (PARI) list(lim)=my(v=List(), t); forprime(b=5, , if(3*b*(b^2+81)/2>lim, break); forprime(a=3, b-2, my(c=(a^4+b^2)/2, t=a*b*c); if(t>lim, break); if(isprime(c), listput(v, t)))); Set(v) \\ Charles R Greathouse IV, Aug 29 2015 CROSSREFS Cf. A261656, A261658. Sequence in context: A075667 A136543 A133274 * A086393 A336943 A108251 Adjacent sequences:  A261654 A261655 A261656 * A261658 A261659 A261660 KEYWORD nonn AUTHOR David Ferris, Aug 28 2015 STATUS approved

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Last modified October 21 02:29 EDT 2021. Contains 348141 sequences. (Running on oeis4.)