|
|
A106551
|
|
a(n) = n-th semiprime + (concatenation of its two divisors, largest divisor first).
|
|
1
|
|
|
26, 38, 42, 62, 86, 68, 94, 134, 80, 158, 146, 206, 110, 230, 172, 278, 126, 224, 170, 250, 350, 374, 200, 302, 446, 194, 494, 260, 518, 380, 228, 406, 566, 290, 638, 484, 350, 710, 296, 1232, 734, 536, 562, 330, 806, 614, 854, 1454, 440, 878, 470, 950, 692
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,1
|
|
COMMENTS
|
Concatenation of the divisors starting with the smallest one leads to a different sequence.
|
|
LINKS
|
|
|
EXAMPLE
|
First semiprime is 4; 4 is 2*2; 26=4+22.
Second semiprime is 6; 6 is 3*2; 38=6+32.
Third semiprime is 9; 9 is 3*3; 42=9+33.
Fourth semiprime is 10; 10 is 5*2; 62=10+52.
|
|
MAPLE
|
N:= 1000: # for terms <= N
Primes:= select(isprime, [2, seq(i, i=3..N/2, 2)]):
R:= NULL:
for i from 1 to nops(Primes) do for j from 1 to i while Primes[i]*Primes[j] <= N do
R:= R, [Primes[i]*Primes[j], Primes[i]*10^(1+ilog10(Primes[j]))+Primes[j]]
od od:
map(convert, sort([R], (x, y) -> x[1]<y[1]), `+`); # Robert Israel, Jun 09 2020
|
|
MATHEMATICA
|
cp[n_] := Block[{p = Reverse[ First /@ FactorInteger[ n]]}, If[ Length[p] == 1, p = Join[p, p]]; n + FromDigits[ Join @@ IntegerDigits@p]]; cp /@ Select[ Range@ 160, 2 == PrimeOmega@ # &] (* Giovanni Resta, Jun 10 2020 *)
|
|
CROSSREFS
|
|
|
KEYWORD
|
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|