A194156 Prime number that appears the most often as the n-th prime factor of an integer in a factorization given in ascending order.

13, 23, 47, 113, 199, 283, 467, 887, 1627, 2803, 4297, 6397, 10343, 18461, 29453, 43067, 67993, 102679, 155893, 267961, 395323, 617819, 926707, 1513751, 2160469, 3278837, 4991687, 7115989, 11113793, 16310629, 24417233, 33888653, 52100569, 76020569

Offset

4,1

Comments

a(1) = 2 and a(2) = 3. The table in Koninck's book has 5 and 7 tied for third place.

References

J.-M. De Koninck, Those Fascinating Numbers, Amer. Math. Soc., 2009, page 56.

Links

Table of n, a(n) for n=4..37.

Jon E. Schoenfield, Magma code

Formula

a(n) = prime(k) k >= n, such that A096294(k-1, n-1)/A002110(k) >= A096294(i-1, n-1)/A002110(i) for i >= n, i <> k. - Peter Munn, Jul 31 2019

Example

a(1) = 2 because, since a randomly chosen number has a 50% chance of being even, the first prime in the factorization of an integer is most likely to be 2.
a(4) = 13 because in the factorization of a number with four or more prime factors, 13 is likeliest to be fourth.

Mathematica

a[n_] := a[n] = (ClearAll[ CurrPrimes, t]; d = 1.0; For[i = 1, i <= n, i++, CurrPrimes[i] = Prime[i]; d = d*CurrPrimes[i]; t[i] = 1.0]; Freq = t[n]/d; k = n; FreqMax1 = Freq; kAtFreqMax1 = k; While[ k <= kAtFreqMax1*2 , k = k+1;  t[1] = t[1]*(CurrPrimes[1] - 1); CurrPrimes[1] = CurrPrimes[2]; For[i = 2, i <= n, i++,  t[i] = t[i]*(CurrPrimes[i] - 1) + t[i-1]; CurrPrimes[i-1] = CurrPrimes[i]]; CurrPrimes[n] = NextPrime[ CurrPrimes[n-1]]; d = d*CurrPrimes[n]; Freq = t[n]/d; If[ Freq > FreqMax1 , FreqMax1 = Freq; kAtFreqMax1 = k]; If[ Mod[k, 100] == 0 || (CurrPrimes[n] == 16111) , k,  CurrPrimes[n], Freq]; (*end while*)];  Prime[kAtFreqMax1] ); A194156 = Table[ Print["a(", n, ") = ", a[n]]; a[n], {n, 4, 30}] (* Jean-François Alcover, Dec 14 2011, translated from Jon E. Schoenfield's Magma code *)

Prog

See link above.

Crossrefs

Cf. A096294, A284411.

Sequence in context: A068712 A103166 A154863 * A139863 A304672 A316119

Adjacent sequences: A194153 A194154 A194155 * A194157 A194158 A194159

Keyword

nonn,nice

Author

Alonso del Arte, Aug 17 2011

Extensions

Corrected and extended from a(19) onwards by Jon E. Schoenfield, who says (Start):

a(13) was given as 2083, but should be 2803 (apparent typo).

a(17) was given as 16111, which is the most-frequently-appearing 17th prime factor among any primes *up to that point*, as well as for a considerable distance beyond it ... but (after a significant gap) there are larger primes that appear as the 17th prime factor with a slightly higher frequency; beyond 16111, new record highs occur at 18257, 18311, 18313, 18457, and 18461 (the last of which is the correct value for a(17)).

a(18) was given as 24251, but a similar situation applies there; it's the most-frequently-appearing 18th prime factor among any primes up to that point, but it's beaten out by 27109, which is the best until 29443, which is the best until 29453 (which is the correct value for a(18)). (End)

Status

approved