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 A228164 Numbers n having at least two distinct symmetrical pairs of divisors (a, b) and (b', a') such that n = a*b = b'*a' with a' = reverse(a) and b' = reverse(b). 2
 504, 756, 806, 1008, 1148, 1209, 1472, 1512, 2016, 2208, 2418, 2772, 2924, 3024, 4416, 4433, 5544, 6314, 8096, 8316, 8415, 8866, 10736, 11088, 12628, 13277, 13299, 14300, 16038, 16082, 16192, 16632, 17732, 20405, 21384, 22176, 24288, 24948, 25452, 26598, 26730 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A pair of integers (a, b) is symmetrical for multiplication when the product a*b is the same as the product b'*a' where a' = reverse(a) and b' = reverse(b). A double pair shows a symmetrical structure, for example: 23*64 = 46*32; 42*36 = 63*24; 21*36 = 63*12; 21*48 = 84*12; 31*26 = 62*13. Because it is possible to obtain a number of double pairs equal to 1, 2, 3, ... we introduce the notion of "symmetrical order" denoted So(n) for each number n of the sequence corresponding to the number of double pairs. The numbers of the sequence n = 50904, 55944, 76356, 81406, 83916, ... generate two double pairs of the form (a, b) and (b', a'), (c, d) and (d', c') such that n = a*b = b'*a' with a' = reverse(a) and b' = reverse(b) and n = c*d = d'*c' with c' = reverse(c) and d' = reverse(d). Hence So(50904) = 2, So(55944) = 2, ... The number n = 101808 implies So(n) = 3 because this number generates 3 double couples (see the example below). The sequence shows primitive and nonprimitive values: for example n = 504, 756, 806, ... are primitive values, but n = 1008 = 2*504, 1512 = 2*756, 2016 = 4*504, ... are not primitive values. A primitive number contains a couple of divisors (a, b) where a (and/or) b has decimal digits less than 5. REFERENCES David Wells, The Penguin Dictionary of Curious and Interesting Numbers, 2nd Ed. (1997), p. 142. LINKS Michael De Vlieger, Table of n, a(n) for n = 1..1000 Michael De Vlieger, Symmetrical divisor pairs for numbers m in A228164 with 1 <= m <= 10^7. EXAMPLE 504 is in the sequence because the two pairs of divisors (42, 12) and (21, 24) have the property 42*12 = 21*24 = 504 with 42 = reverse(24) and 12 = reverse(21). 50904 is in the sequence because we obtain two double pairs of divisors: (12, 4242) and (2424, 21), (42, 1212) and (2121, 24); 101808 is in the sequence because we obtain three double pairs of divisors: (12, 8484) and (4848, 21), (24, 4242) and (2424, 42), (48, 2121) and (1212, 84). From Michael De Vlieger, Sep 15 2017: (Start) First positions of numbers k of symmetrical pairs that appear for a(n) <= 10^7. k     n     a(n) ---------------- 2     1      504 3     4     1008 4    17     5544 6    98   101808 8   274   559944 (End) MAPLE with(numtheory):for n from 2 to 50000 do:x:=divisors(n):n1:=nops(x):ii:=0:for a from 2 to n1-1 while(ii=0) do:m:=n/x[a]:m1:=convert(m, base, 10):nn1:=nops(m1): m2:=convert(x[a], base, 10):nn2:=nops(m2): s1:=sum('m1[nn1-i+1]*10^(i-1)', 'i'=1..nn1): s2:=sum('m2[nn2-i+1]*10^(i-1)', 'i'=1..nn2):for b from a+1 to n1-1 while(ii=0) do:q:=n/x[b]:if s1=q and s2=x[b] and m<>x[b] then ii:=1:printf(`%d, `, n):else fi:od:od:od: MATHEMATICA Select[Range[10^7], Function[n, Count[Rest@ Select[Divisors@ n, # <= Sqrt@ n &], _?(And[IntegerReverse@ # != #, IntegerReverse@ # IntegerReverse[n/#] == n] &)] > 1]] (* Michael De Vlieger, Oct 09 2015, updated Sep 15 2017 *) CROSSREFS Cf. A262873 (a subsequence of predestined numbers A262743). Sequence in context: A043304 A045212 A262873 * A060666 A335654 A226265 Adjacent sequences:  A228161 A228162 A228163 * A228165 A228166 A228167 KEYWORD nonn,base AUTHOR Michel Lagneau, Aug 17 2013 STATUS approved

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Last modified June 26 19:51 EDT 2022. Contains 354885 sequences. (Running on oeis4.)