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A037274 Home primes: for n >= 2, a(n) = the prime that is finally reached when you start with n, concatenate its prime factors (A037276) and repeat until a prime is reached (a(n) = -1 if no prime is ever reached). 60

%I

%S 1,2,3,211,5,23,7,3331113965338635107,311,773,11,223,13,13367,1129,

%T 31636373,17,233,19,3318308475676071413,37,211,23,331319,773,3251,

%U 13367,227,29,547,31,241271,311,31397,1129,71129,37,373,313,3314192745739,41,379,43,22815088913,3411949,223,47,6161791591356884791277

%N Home primes: for n >= 2, a(n) = the prime that is finally reached when you start with n, concatenate its prime factors (A037276) and repeat until a prime is reached (a(n) = -1 if no prime is ever reached).

%C The initial 1 could have been omitted.

%C Probabilistic arguments give exactly zero for the chance that the sequence of integers starting at n contains no prime, the expected number of primes being given by a divergent sequence. - _J. H. Conway_

%C After over 100 iterations, a(49) is still composite - see A056938 for the latest information.

%C More terms:

%C a(50) to a(60) are 3517, 317, 2213, 53, 2333, 773, 37463, 1129, 229, 59, 35149;

%C a(61) to a(65) are 61, 31237, 337, 1272505013723, 1381321118321175157763339900357651;

%C a(66) to a(76) are 2311, 67, 3739, 33191, 257, 71, 1119179, 73, 379, 571, 333271.

%C This is different from A195264. Here 8 = 2^3 -> 222 -> ... -> 3331113965338635107 (a prime), whereas in A195264 8 = 2^3 -> 23 (a prime). - _N. J. A. Sloane_, Oct 12 2014

%D Jeffrey Heleen, Family Numbers: Mathemagical Black Holes, Recreational and Educational Computing, 5:5, pp. 6, 1990.

%D Jeffrey Heleen, Family numbers: Constructing Primes by Prime Factor Splicing, J. Recreational Math., Vol. 28 #2, 1996-97, pp. 116-119.

%H Christian N. K. Anderson, <a href="/A037274/a037274_1.txt">Table of known values of n, # of steps to reach a(n), and a(n)</a> or NA if a(n) has 30 digits or more. Also, the trajectory, with factors separated by a |, terminated by either "(end)" or "-> ?" if a(n) has 30 digits or more.

%H P. De Geest, <a href="http://www.worldofnumbers.com/topic1.htm">Home Primes < 100 and Beyond</a>

%H M. Herman and J. Schiffman, <a href="http://www.jstor.org/stable/10.5951/mathteacher.107.8.0606">Investigating home primes and their families</a>, Math. Teacher, 107 (No. 8, 2014), 606-614.

%H N. J. A. Sloane, <a href="/A195264/a195264.pdf">Confessions of a Sequence Addict (AofA2017)</a>, slides of invited talk given at AofA 2017, Jun 19 2017, Princeton. Mentions this sequence.

%H N. J. A. Sloane, Three (No, 8) Lovely Problems from the OEIS, Experimental Mathematics Seminar, Rutgers University, Oct 05 2017, <a href="https://vimeo.com/237029685">Part I</a>, <a href="https://vimeo.com/237030304">Part 2</a>, <a href="https://oeis.org/A290447/a290447_slides.pdf">Slides.</a> (Mentions this sequence)

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/HomePrime.html">Home Prime.</a>

%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Home_prime">Home prime</a>

%e 9 = 3*3 -> 33 = 3*11 -> 311, prime, so a(9) = 311.

%e The trajectory of 8 is more interesting:

%e 8 ->

%e 2 * 2 * 2 ->

%e 2 * 3 * 37 ->

%e 3 * 19 * 41 ->

%e 3 * 3 * 3 * 7 * 13 * 13 ->

%e 3 * 11123771 ->

%e 7 * 149 * 317 * 941 ->

%e 229 * 31219729 ->

%e 11 * 2084656339 ->

%e 3 * 347 * 911 * 118189 ->

%e 11 * 613 * 496501723 ->

%e 97 * 130517 * 917327 ->

%e 53 * 1832651281459 ->

%e 3 * 3 * 3 * 11 * 139 * 653 * 3863 * 5107

%e and 3331113965338635107 is prime, so a(8) = 3331113965338635107.

%t f[n_] := FromDigits@ Flatten[ IntegerDigits@ Table[ #[[1]], { #[[2]] }] & /@ FactorInteger@n, 2]; g[n_] := NestWhile[ f@# &, n, !PrimeQ@# &]; g[1] = 1; Array[g, 41] (* _Robert G. Wilson v_, Sep 22 2007 *)

%o (PARI) step(n)=my(f=factor(n),s="");for(i=1,#f~,for(j=1,f[i,2],s=Str(s,f[i,1]))); eval(s)

%o a(n)=if(n<4,return(n)); while(!isprime(n), n=step(n)); n \\ _Charles R Greathouse IV_, May 14 2015

%o (SageMath)

%o def digitLen(x,n):

%o r=0

%o while(x>0):

%o x//=n

%o r+=1

%o return r

%o def concatPf(x,n):

%o r=0

%o f=list(factor(x))

%o for c in xrange(len(f)):

%o for d in xrange(f[c][1]):

%o r*=(n**digitLen(f[c][0],n))

%o r+=f[c][0]

%o return r

%o def hp(x,n):

%o x1=concatPf(x,n)

%o while(x1!=x):

%o x=x1

%o x1=concatPf(x1,n)

%o return x

%o #example: prints the home prime of 8 in base 10

%o print(hp(8,10))

%Y Cf. A006919, A037271, A037272, A037273, A037275, A037276, A037919-A037941, A048986, A056938.

%Y Cf. A195264 (use exponents instead of repeating primes).

%Y Cf. A084318 (use only one copy of each prime), A248713 (Fermi-Dirac analog: use unique representation of n>1 as a product of distinct terms of A050376).

%Y Cf. also A120716 and related sequences.

%K nonn,nice,base

%O 1,2

%A _N. J. A. Sloane_, _Jeff Burch_

%E Corrected and extended by _Karl W. Heuer_, Sep 30 2003

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Last modified February 23 17:23 EST 2018. Contains 299584 sequences. (Running on oeis4.)