A242726 Sphenic numbers k = p*q*r such that reversal(k) is also a sphenic number and reversal(k) = reversal(p)*reversal(q)*reversal(r).
66, 286, 606, 682, 2222, 2486, 2626, 2882, 3333, 3939, 5555, 6262, 6842, 6886, 7777, 9393, 14443, 18887, 22462, 22682, 22826, 24266, 26422, 26462, 26686, 28622, 33693, 34441, 36399, 39633, 39693, 62822, 66242, 68662, 78881, 99363, 118877, 125543, 145541
Offset: 1
Examples
3196751 = 31*101*1021 is in the sequence because reversal(3196751) = 1576913 = 13*101*1201 => 31 = reversal(13), 101 = reversal(101) and 1201 = reversal(1021).
Links
- Chai Wah Wu, Table of n, a(n) for n = 1..2175
Programs
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Maple
with(numtheory): for n from 30 to 150000 do : x:=factorset(n):n1:=nops(x): if bigomega(n)= 3 and n1>2 then y:=convert(n,base,10):n2:=nops(y): p:=x[1]:q:=x[2]:r:=x[3]: xp1:=convert(p,base,10):nxp1:=nops(xp1): xq1:=convert(q,base,10):nxq1:=nops(xq1): xr1:=convert(r,base,10):nxr1:=nops(xr1): sp:=sum('xp1[i]*10^(nxp1-i)', 'i'=1..nxp1): sq:=sum('xq1[i]*10^(nxq1-i)', 'i'=1..nxq1): sr:=sum('xr1[i]*10^(nxr1-i)', 'i'=1..nxr1): lst:={sp} union {sq} union {sr}: s:=sum('y[i]*10^(n2-i)', 'i'=1..n2):x1:=factorset(s):nn1:=nops(x1): if bigomega(s)=3 and nn1>2 then z:=convert(s,base,10):n3:=nops(z): p1:=x1[1]:q1:=x1[2]:r1:=x1[3]: lst1:={p1} union {q1} union {r1}: s1:=sum('z[i]*10^(n3-i)','i'=1..n3): if lst = lst1 then printf(`%d, `,n): else fi: fi: fi: od:
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