|
Search: id:A133608
|
|
|
| A133608 |
|
Numbers n such that the sum of digits of n-th semiprime equals sum of digits of n. |
|
+0
1
|
|
| 5, 6, 19, 40, 41, 42, 70, 71, 85, 89, 128, 148, 149, 166, 199, 246, 257, 271, 285, 327, 339, 346, 448, 449, 469, 484, 566, 592, 605, 617, 634, 643, 644, 676, 682, 694, 710, 713, 719, 740, 748, 751, 752, 753, 782, 793, 794, 797, 798, 815, 890, 901, 905, 961
(list; graph; listen)
|
|
|
OFFSET
|
1,1
|
|
|
COMMENT
|
This is to A033549 as semiprimes A001358 are to primes A000040.
|
|
FORMULA
|
A007953(A001358(a(n))) = A007953(a(n)).
|
|
EXAMPLE
|
a(1) = 5 because semiprime(5) = 14, whose sum of digits is 5, the same as its index as a semiprime.
|
|
MATHEMATICA
|
a = {}; c = 0; For[n = 4, n < 10000, n++, If[Sum[FactorInteger[n][[i, 2]], {i, 1, Length[FactorInteger[n]]}] == 2, c++; If[Plus @@ IntegerDigits[c] == Plus @@ IntegerDigits[n], AppendTo[a, c]]]]; a - Stefan Steinerberger (stefan.steinerberger(AT)gmail.com), Dec 29 2007
SemiPrimePi[n_] := Sum[ PrimePi[n/Prime@i] - i + 1, {i, PrimePi@ Sqrt@n}]; SemiPrime[n_] := Block[{e = Floor[Log[2, n] + 1], a, b}, a = 2^e; Do[b = 2^p; While[SemiPrimePi@a < n, a = a + b]; a = a - b/2, {p, e, 0, -1}]; a + b/2]; Select[Range@ 1000, fQ@# &] (* Robert G. Wilson v *)
|
|
CROSSREFS
|
Cf. A001358, A007953, A033549.
Sequence in context: A163772 A056509 A129722 this_sequence A072577 A057520 A060423
Adjacent sequences: A133605 A133606 A133607 this_sequence A133609 A133610 A133611
|
|
KEYWORD
|
base,easy,nonn,less
|
|
AUTHOR
|
Jonathan Vos Post (jvospost3(AT)gmail.com), Dec 27 2007
|
|
EXTENSIONS
|
Corrected and extended by Stefan Steinerberger (stefan.steinerberger(AT)gmail.com) and Robert G. Wilson v (rgwv(AT)rgwv.com), Dec 29 2007
|
|
|
Search completed in 0.002 seconds
|
