

A050376


Numbers of the form p^(2^k) where p is prime and k >= 0.


121



2, 3, 4, 5, 7, 9, 11, 13, 16, 17, 19, 23, 25, 29, 31, 37, 41, 43, 47, 49, 53, 59, 61, 67, 71, 73, 79, 81, 83, 89, 97, 101, 103, 107, 109, 113, 121, 127, 131, 137, 139, 149, 151, 157, 163, 167, 169, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241
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OFFSET

1,1


COMMENTS

Every number n is a product of a unique subset of these numbers: in the prime factorization n = prod( j>=1, p(j)^e(j) ) expand every exponent e(j) as binary number and pick the terms of this sequence corresponding to the positions of the ones in binary (it is clear that both n and n^2 have the same number of factors in this sequence, and that each factor appears with exponent 1 or 0).
Or, a(1) = 2; for n>1, a(n) = smallest number which cannot be obtained as the product of previous terms. This is evident from the unique factorization theorem and the fact that every number can be expressed as the sum of powers of 2.  Amarnath Murthy, Jan 09 2002
Except for the first term, same as A084400.  David Wasserman, Dec 22 2004
The least number having 2^n divisors (=A037992(n)) is the product of the first n terms of this sequence according to Ramanujan.
According to the BoseEinstein distribution of particles, an unlimited number of particles may occupy the same state. On the other hand, according to the FermiDirac distribution, no two particles can occupy the same state (by the Pauli exclusion principle). Unique factorizations of the positive integers by primes (A000040) and over terms of A050376 one can compare with two these distributions in physics of particles. In the correspondence with this, the factorizations over primes one can call "BoseEinstein factorizations", while the factorizations over distinct terms of A050376 one can call "FermiDirac factorizations".  Vladimir Shevelev, Apr 16 2010
The numbers of the form p^(2^k), where p is prime and k >= 0, might thus be called the "FermiDirac primes", while the classic primes might be called the "BoseEinstein primes".  Daniel Forgues, Feb 11 2011
In the theory of infinitary divisors, the most natural name of the terms is "infinitary primes" or "iprimes". Indeed, n is in the sequence, if and only if it has only two infinitary divisors. Since 1 and n are always infinitary divisors of n>1, an iprime has no other infinitary divisors.  Vladimir Shevelev, Feb 28 2011
{a(n)} is the minimal set including all primes and closed with respect to squaring. In connection with this, note that n and n^2 have the same number of factors in their FermiDirac representations.  Vladimir Shevelev, Mar 16 2012
In connection with this sequence, call an integer compact if the factors in its FermiDirac factorization are pairwise coprime. The density of such integers equals (6/pi^2)*prod{prime p}(1+(sum{i>=1} p^((2^i1))/(p+1))=0.872497... It is interesting that there exist only 7 compact factorials listed in A169661.  Vladimir Shevelev, Mar 17 2012
The first k terms of the sequence solve the following optimization problem:
Let x_1, x_2,..., x_k be integers with the restrictions: 2<=x_1<x_2<...<x_k, A064547(prod{i=1,...,k} x_i) >= k. Let the goal function be prod{i=1,...,k} x_i. Then minimal value of the goal function is prod{i=1,...,k} a(i).  Vladimir Shevelev, Apr 01 2012
From Joerg Arndt, Mar 11 2013: (Start)
Similarly to the first comment, for the sequence "Numbers of the form p^(3^k) or p^(2*3^k) where p is prime and k >= 0" one obtains a factorization into distinct factors by using the ternary expansion of the exponents (here n and n^3 have the same number of such factors).
The generalization to base r would use "Numbers of the form p^(r^k), p^(2*r^k), p^(3*r^k), ..., p^((r1)*r^k) where p is prime and k >= 0" (here n and n^r have the same number of (distinct) factors). (End)
The first appearance of this sequence as a multiplicative basis in number theory with some new notions, formulas and theorems may have been in my 1981 paper (see the Abramovich reference).  Vladimir Shevelev, Apr 27 2014
Numbers n for which A064547(n) = 1.  Antti Karttunen, Feb 10 2016


REFERENCES

V. S. Abramovich, On an analog of the Euler function, Proceeding of the NorthCaucasus Center of the Academy of Sciences of the USSR (Rostov na Donu) (1981) No. 2, 1317 (Russian; MR0632989(83a:10003)).
S. Ramanujan, Highly Composite Numbers, Collected Papers of Srinivasa Ramanujan, p. 125, Ed. G. H. Hardy et al., AMS Chelsea 2000.
V. S. Shevelev, Multiplicative functions in the FermiDirac arithmetic, Izvestia Vuzov of the NorthCaucasus region, Nature sciences 4 (1996), 2843 (in Russian; MR 2000f: 11097, pp. 39123913).


LINKS

T. D. Noe and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
Steven R. Finch, Unitarism and Infinitarism, February 25, 2004. [Cached copy, with permission of the author]
Simon Litsyn and Vladimir Shevelev, On factorization of integers with restrictions on the exponent, INTEGERS: Electronic Journal of Combinatorial Number Theory, 7 (2007), #A33, 136.
OEIS Wiki, "FermiDirac representation" of n.
V. Shevelev, Compact integers and factorials, Acta Arith. 126 (2007), no.3, 195236.


FORMULA

From Vladimir Shevelev, Mar 16 2012 (Start)
Prod_(i>=1) a(i)^k_i = n!, where k_i = floor(n/a(i))floor(n/a(i)^2)+floor(n/a(i)^3)floor(n/a(i)^4)+...
Denote by A(x) the number of terms not exceeding x.
Then A(x) = pi(x)+pi(x^(1/2))+pi(x^(1/4))+pi(x^(1/8))+...
Conversely, pi(x) = A(x)A(sqrt(x)). For example, pi(37) = A(37)A(6) = 164 = 12.
(End)
A209229(A100995(a(n))) = 1.  Reinhard Zumkeller, Mar 19 2013
A FermiDirac analog of Euler product: for s>1, Zeta(s) = prod{k>=1}(1+a(k)^(s)).
In particular, prod{k>=1}(1+a(k)^(2)) = pi^2/6.  Vladimir Shevelev, Aug 31 2013
a(n) = A268385(A268392(n)). [By their definitions.]  Antti Karttunen, Feb 10 2016
A000040 union A001248 union A030514 union A179645 union A030635 union ....  R. J. Mathar, May 26 2017


EXAMPLE

Prime powers which are not members of this sequence:
8 = 2^3 = 2^(1+2), 27 = 3^3 = 3^(1+2), 32 = 2^5 = 2^(1+4),
64 = 2^6 = 2^(2+4), 125 = 5^3 = 5^(1+2), 128 = 2^7 = 2^(1+2+4)
"FermiDirac factorizations":
6 = 2*3, 8 = 2*4, 24 = 2*3*4, 27 = 3*9, 32 = 2*16, 64 = 4*16,
108 = 3*4*9, 120 = 2*3*4*5, 121 = 121, 125 = 5*25, 128 = 2*4*16.


MAPLE

isA050376 := proc(n)
local f, e;
f := ifactors(n)[2] ;
if nops(f) = 1 then
e := op(2, op(1, f)) ;
if isA000079(e) then
true;
else
false;
end if;
else
false;
end if;
end proc:
A050376 := proc(n)
option remember ;
local a;
if n = 1 then
2 ;
else
for a from procname(n1)+1 do
if isA050376(a) then
return a;
end if;
end do:
end if;
end proc: # R. J. Mathar, May 26 2017


MATHEMATICA

nn = 300; t = {}; k = 1; While[lim = nn^(1/k); lim > 2, t = Join[t, Prime[Range[PrimePi[lim]]]^k]; k = 2 k]; t = Union[t] (* T. D. Noe, Apr 05 2012 *)


PROG

(PARI) {a(n)= local(m, c, k, p); if(n<=0, 2*(n==0), c=0; m=2; while( c<n, m++; if( isprime(m) ( (k=ispower(m, (null), &p))&isprime(p)& k ==2^valuation(k, 2) ), c++)); m)} /* Michael Somos, Apr 15 2005 */
(PARI) lst(lim)=my(v=primes(primepi(lim)), t); forprime(p=2, sqrt(lim), t=p; while((t=t^2)<=lim, v=concat(v, t))); vecsort(v) \\ Charles R Greathouse IV, Apr 10 2012
(PARI) is_A050376(n)=2^#binary(n=isprimepower(n))==n*2 \\ M. F. Hasler, Apr 08 2015
(PARI) ispow2(n)=n && n>>valuation(n, 2)==1
is(n)=ispow2(isprimepower(n)) \\ Charles R Greathouse IV, Sep 18 2015
(Haskell)
a050376 n = a050376_list !! (n1)
a050376_list = filter ((== 1) . a209229 . a100995) [1..]
 Reinhard Zumkeller, Mar 19 2013
(Scheme)
(define A050376 (MATCHINGPOS 1 1 (lambda (n) (= 1 (A064547 n)))))
;; Requires also my IntSeqlibrary.  Antti Karttunen, Feb 09 2016


CROSSREFS

Cf. A000040 (primes, is a subsequence), A026416, A026477, A050377A050380, A052330, A064547, A066724, A084400, A176699, A182979.
Cf. A268388 (complement without 1).
Cf. A124010, subsequence of A000028, A000961, A213925, A223490.
Cf. A228520, A186945 (FermiDirac analog of Ramanujan primes, A104272, and Labos primes, A080359).
Cf. also A268385, A268391, A268392.
Sequence in context: A026477 A079852 A084400 * A280257 A050198 A158923
Adjacent sequences: A050373 A050374 A050375 * A050377 A050378 A050379


KEYWORD

nonn,easy,nice


AUTHOR

Christian G. Bower, Nov 15 1999


EXTENSIONS

Edited by Charles R Greathouse IV, Mar 17 2010
More examples from Daniel Forgues, Feb 09 2011


STATUS

approved



