A251361
Numbers k such that pi(k) is the concatenation of distinct prime factors of k, in increasing order.
Original entry on oeis.org
4, 100, 31509, 7560625
Offset: 1
4 is in the sequence since 4=2^2 and pi(4)=2,
100 is in the sequence since 100=2^2*5^2 and pi(100)=25,
31509 is in the sequence since 31509=3^4*389 and pi(31509)=3389, and
7560625 is in the sequence since 7560625=5^4*12097 and pi(7560625)=512097.
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a251361[n_Integer] := Select[Range[n], PrimePi[#] == FromDigits[
Flatten@ IntegerDigits[First@ Transpose@ FactorInteger[#]]] &]; a251361[10^6] (* Michael De Vlieger, Dec 03 2014 *)
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is(n)=eval(fold((x,y)->Str(x,y),factor(n)[,1]))==primepi(n) \\ Charles R Greathouse IV, Dec 06 2014
A251362
Numbers n such that n is the concatenation of distinct prime factors of phi(n), in increasing order.
Original entry on oeis.org
25, 235741, 23517131, 274873357929, 2357131984859
Offset: 1
25 is in the sequence since phi(25)=2^2*5,
235741 is in the sequence since phi(235741)=2^4*3^2*5*7*41,
23517131 is in the sequence since phi(23517131)=2^7*3*5^2*17*131.
A251363
Numbers n such that n is the concatenation of distinct prime factors of phi(n), in decreasing order.
Original entry on oeis.org
237532, 832332, 82953292, 423238803752
Offset: 1
237532 is in the sequence since phi(237532)=23*7*5*3^2*2^4,
832332 is in the sequence since phi(832332)=83*23*3^2*2^4, and
82953292 is in the sequence since phi(82953292)=829*53*29*2^5.
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a251363[n_Integer] :=
Rest@ Select[Range[n], # == FromDigits[Flatten@ IntegerDigits[
Sort[First@ Transpose@ FactorInteger[EulerPhi[#]], Greater]]] &]; a251363[10^6] (* Michael De Vlieger, Dec 03 2014 *)
Showing 1-3 of 3 results.
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