A176506 - OEIS

A176506

Difference between the prime indices of the two factors of the n-th semiprime.

0, 1, 0, 2, 3, 1, 2, 4, 0, 5, 3, 6, 1, 7, 4, 8, 0, 5, 2, 6, 9, 10, 3, 7, 11, 1, 12, 4, 13, 8, 2, 9, 14, 5, 15, 10, 6, 16, 3, 0, 17, 11, 12, 4, 18, 13, 19, 1, 7, 20, 8, 21, 14, 5, 22, 0, 15, 23, 16, 9, 2, 24, 17, 25, 6, 10, 26, 3, 18, 27, 11, 7, 28, 19, 1, 29, 12, 20, 2, 21, 4, 30, 8, 31, 13, 22

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,4

COMMENTS

Are there no adjacent equal terms? I have verified this up to n = 10^6. - Gus Wiseman, Dec 04 2020

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A049084(A084127(n)) - A049084(A084126(n)). [corrected by R. J. Mathar, Apr 22 2010]

a(n) = A338913(n) - A338912(n). - Gus Wiseman, Dec 04 2020

EXAMPLE

From Gus Wiseman, Dec 04 2020: (Start)

The sequence of semiprimes together with the corresponding differences begins:

4: 1 - 1 = 0

6: 2 - 1 = 1

9: 2 - 2 = 0

10: 3 - 1 = 2

14: 4 - 1 = 3

15: 3 - 2 = 1

21: 4 - 2 = 2

22: 5 - 1 = 4

25: 3 - 3 = 0

26: 6 - 1 = 5

33: 5 - 2 = 3

(End)

MAPLE

isA001358 := proc(n) numtheory[bigomega](n) = 2 ; end proc:

A001358 := proc(n) option remember ; if n = 1 then return 4 ; else for a from procname(n-1)+1 do if isA001358(a) then return a; end if; end do; end if; end proc:

A084126 := proc(n) min(op(numtheory[factorset](A001358(n)))) ; end proc:

A084127 := proc(n) max(op(numtheory[factorset](A001358(n)))) ; end proc:

A176506 := proc(n) numtheory[pi](A084127(n)) - numtheory[pi](A084126(n)) ; end proc: seq(A176506(n), n=1..120) ; # R. J. Mathar, Apr 22 2010

# Alternative:

N:= 500: # to use the first N semiprimes

Primes:= select(isprime, [2, seq(i, i=3..N/2, 2)]):

SP:= NULL:

for i from 1 to nops(Primes) do

for j from 1 to i do

sp:= Primes[i]*Primes[j];

if sp > N then break fi;

SP:= SP, [sp, i-j]

od od:

SP:= sort([SP], (s, t) -> s[1]<t[1]):

map(t -> t[2], SP); # Robert Israel, Jan 17 2019

MATHEMATICA

M = 500; (* to use the first M semiprimes *)

primes = Select[Join[{2}, Range[3, M/2, 2]], PrimeQ];

SP = {};

For[i = 1, i <= Length[primes], i++,

For[j = 1, j <= i, j++,

sp = primes[[i]] primes[[j]];

If[sp > M, Break []];

AppendTo[SP, {sp, i - j}]

]];

SortBy[SP, First][[All, 2]] (* Jean-François Alcover, Jul 18 2020, after Robert Israel *)

Table[If[!SquareFreeQ[n], 0, -Subtract@@PrimePi/@First/@FactorInteger[n]], {n, Select[Range[100], PrimeOmega[#]==2&]}] (* Gus Wiseman, Dec 04 2020 *)

PROG

(PARI) lista(nn) = {my(vsp = select(x->(bigomega(x)==2), [1..nn])); vector(#vsp, k, my(f=factor(vsp[k])[, 1]); primepi(vecmax(f)) - primepi(vecmin(f))); } \\ Michel Marcus, Jul 18 2020

CROSSREFS

Cf. A109313.

Cf. A001358, A049084, A084126, A084127.

A087794 is product of the same indices.

A176504 is the sum of the same indices.

A115392 lists positions of first appearances.

A128301 lists positions of 0's.

A172348 lists positions of 1's.

A338898 has this sequence as row differences.

A338900 is the squarefree case.

A338912/A338913 give the two prime indices of semiprimes.

A006881 lists squarefree semiprimes.