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A337523
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Numbers of the form ab such that uphi(ab) = a*b where ab is the concatenation of a and b.
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0
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18, 26, 68, 87, 154, 165, 209, 572, 846, 1434, 4840, 5476, 5828, 5936, 6499, 6572, 7772, 8540, 8727, 10088, 10864, 11772, 12867, 15088, 20099, 20584, 20881, 21672, 22440, 27348, 29748, 29920, 30576, 32390, 35640, 36580, 37200, 37449, 38430, 39600, 40548, 42984
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OFFSET
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1,1
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LINKS
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EXAMPLE
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For k = 18, uphi(18) = 8 = 1 * 8.
For k = 68, uphi(68) = 48 = 6 * 8.
For k = 87, uphi(87) = 56 = 8 * 7.
For k = 154, uphi(154) = 60 = 15 * 4.
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MATHEMATICA
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f[p_, e_] := p^e - 1; uphi[1] = 1; uphi[n_] := Times @@ f @@@ FactorInteger[n]; seqQ[n_] := Module[{d = IntegerDigits[n]}, MemberQ[Times @@@ Table[FromDigits /@ {Take[d, k], Take[d, -Length[d] + k]}, {k, 1, Length[d] - 1}], uphi[n]]]; Select[Range[10, 43000], seqQ] (* Amiram Eldar, Aug 30 2020 *)
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PROG
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(Magma) uphi:=func<n|n eq 1 select 1 else &*[d[1]^d[2]-1: d in Factorization(n)]>; [k:k in [10..43000]| exists(c){i:i in [1..#Intseq(k)-1]| (k mod 10^i)*(k div 10^i) ne 0 and (k mod 10^i)*(k div 10^i) eq uphi(k)}];
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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