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 A261658 Numbers with 4 prime factors a < b < c < d such that 2c = a^4 + b^2 and 2d = b^4 + c^2. 2
 41399193, 195157536843, 548699719043, 3036956318943, 320218213825307, 4132518238158443, 4519695415117057, 6270713759856601, 18507175540175893, 29390150965410193, 106799085933816293, 183320084770933043, 220070939141434093, 481373412121678901 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence is a variation of A261657, using 4 prime factors instead of 3. Some members of this sequence have properties similar to those in A261656. For example, the sequence of divisors of 3*11*101*12421 = 41399193 is approximately linear on a log scale. LINKS EXAMPLE 41399193=3*11*101*12421; 101 = ((3^4)+(11^2))/2 and 12421 = ((11^4)+(101^2))/2, so 41399193 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 d := (b^4+c^2)*(1/2); if isprime(d) then L := [op(L), a*b*c*d]: end if end if end if end do end if end do; L := sort(L) PROG (PARI) factorsm(n)=my(v=factor(n), f=factor(n)[, 1]~, w=[]); for(i=1, #f, for(j=1, v[i, 2], w=concat(w, f[i]))); w; is(n)=f=factorsm(n); if(#f==4, a=f[1]; b=f[2]; c=f[3]; d=f[4]; a

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Last modified December 19 03:45 EST 2018. Contains 318245 sequences. (Running on oeis4.)