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A293652 a(n) is the smallest prime number whose a056240-type is n (see Comments).

Original entry on oeis.org

5, 211, 4327, 4547, 25523, 81611, 966109, 1654111, 3851587, 1895479, 66407189, 134965049, 129312889, 425845151, 35914507, 504365461, 2400397969, 8490141637, 8429770031, 20416021309, 23555107819, 23912414437
Offset: 1

Views

Author

David James Sycamore, Feb 06 2018

Keywords

Comments

For a prime p >= 5 whose prime-index is m, the a056240-type of p is defined to be the unique integer k such that A288814(p) = prime(m-k)*A056240(prime(m)-prime(m-k)).
In other words, k is such that prime(n-k) is the greatest prime divisor of the smallest composite number whose sum of prime factors (taken with multiplicity) is prime(n).
The sequence lists the smallest prime of each successive a056240-type.
In the Examples section, the a056240-type k (=a(k)) of a prime p = prime(m) is indicated by p ~ k(g1,g2,...,gk) where gi = prime(m - i + 1) - prime(m - i). See also A295185.
For the values of the a056240-types of the primes 2, 3, 5, 7, ... see A299912. - N. J. A. Sloane, Mar 10 2018
a(20), a(21) > 14 * 10^9. Conjecture: a(k) > 14 * 10^9 for k > 22. - David A. Corneth, Mar 25 2018
a(20), a(21) computed on the basis of the above conjecture. Note that A321983 records the smallest composite number whose sum of prime divisors (with repetition) is a(n). - David James Sycamore, Nov 30 2018
a(23)..a(25) > 45.8 * 10^9. - David A. Corneth, Dec 02 2018

Examples

			a(1) = 5 since 6 = 3 * 2, the smallest composite number whose prime divisors add to 5, is a multiple of 3, the greatest prime < 5, so k=1; 5 ~ 1(2).
a(2) = 211 since 6501 = 3 * 11 * 197, the smallest composite whose prime divisors add to 211, and 197 < 199 < 211 is the second prime below 211, so k=2, and 211 ~ 2(12,2), and since no smaller prime has this property, a(2)=211.
a(3) = 4327 since 526809 = 3 * 41 * 4283, the smallest composite whose prime divisors add to 4327, and 4283 < 4289 < 4297 < 4327 is the third prime below 4327, so k=3, 4327 ~ 3(30,8,6) and since no smaller prime has this property, a(3)=4327. Likewise,
a(4) = 4547 ~ 4(24, 4, 2, 4),
a(5) = 25523 ~ 5(52, 2, 6, 6, 4),
a(6) = 81611 ~ 6(42, 6, 4, 6, 2, 4),
a(7) = 966109 ~ 7(68, 12, 16, 2, 22, 6, 14),
a(8) = 1654111 ~ 8(54, 14, 4, 6, 2, 4, 6, 2),
a(9) = 3851587 ~ 9(128, 16, 12, 2, 6, 10, 14, 10, 2),
a(10) = 1895479 ~ 10(120, 2, 6, 30, 4, 30, 14, 10, 2, 12),
a(11) = 66407189 ~ 11(120, 6, 6, 16, 14, 6, 4, 8, 10, 2, 4),
a(12) = 134965049 ~ 12(138, 10, 2, 22, 18, 20, 6, 12, 18, 16, 8, 10),
a(13) = 129312889 ~ 13(98, 60, 22, 18, 8, 4, 18, 12, 38, 24, 6, 4, 8),
a(14) = 425845151 ~ 14(148, 2, 42, 16, 50, 24, 12, 6, 4, 20, 6, 48, 10, 12),
a(15) = 35914859 ~ 15(126, 82, 8, 4, 18, 12, 8, 4, 14, 6, 16, 8, 6, 30, 10),
a(16) = 504365461 ~ 16(122, 42, 10, 14, 36, 4, 6, 6, 12, 48, 2, 6, 10, 20, 6, 6),
a(17) = 2400397969 ~ 17(122, 58, 8, 4, 18, 36, 2, 4, 6, 32, 10, 2, 16,12,18,32,12),
a(18) = 8490141637 ~ 18(126, 2, 82, 8, 52, 20, 34, 2, 10, 24, 8, 6,34,2,6,28,24,2),
a(19) = 8429770031 ~ 19(148, 26, 16, 18, 12, 2, 18, 18, 10,20,4,2,6,18,6,4,2,18,4),
a(20) = 20416021309 ~ 20(122, 4, 2, 64, 20, 40, 6, 12, 12, 20, 10, 6, 8, 10, 30, 2, 10, 38, 22, 140,
a(21) = 23555107819 ~ 21(192, 20, 156, 30, 18, 10, 2, 12, 58, 12, 12, 26, 28, 32, 4, 6, 12, 2, 6, 22, 2),
a(22) = 23912414437 ~ 22(344, 4, 12, 14, 40, 2, 4, 18, 2, 36, 10, 12, 2, 10, 26, 10, 24, 14, 40, 30, 14, 12).

Links

Crossrefs

Cf. A056240, A288814, A289993, A295185, A299912, A300334, A300359, A321983.

Programs

  • PARI
    isok(k, n) = my(f=factor(k)); sum(j=1, #f~, f[j, 1]*f[j, 2]) == n;
snumbr(n) = my(k=2); while(!isok(k, n), k++); k; /* A056240 */
scompo(n) = forcomposite(k=4, , if (isok(k, n), return(k))); /* A288814 */
a(n) = {forprime(p=5,,ip = primepi(p); if (ip > n, x = scompo(p); fmax = vecmax(factor(x)[,1]); ifmax = primepi(fmax); if (ip - ifmax == n, y = fmax*snumbr(p - fmax;); if (y == x, return (p);););););} \\ Michel Marcus, Feb 17 2018
  • PARI
    \\ see Corneth link

Extensions

a(7)-a(10) from Michel Marcus, Feb 23 2018
Name changed by N. J. A. Sloane, Mar 10 2018
a(11)-a(19) from David A. Corneth, Mar 24 2018, Mar 25 2018
a(20)-a(21) from David James Sycamore, Nov 30 2018
a(22) from David A. Corneth, Dec 02 2018

A300334 Primes of a056240-type 2.

Original entry on oeis.org

211, 541, 631, 673, 1693, 1801, 2879, 3181, 3271, 3299, 3343, 3571, 3943, 4177, 4441, 4561, 4751, 4783, 4813, 4861, 5147, 5381, 5431, 5501, 5779, 6029, 6197, 6421, 6469, 6521, 6599, 6637, 6883, 7103, 7321, 7369, 7477, 7573, 7603, 7789, 7901, 7963, 8419, 8443, 8641, 8923, 9091, 9587, 9643, 9733, 9781, 9871, 10513
Offset: 1

Views

Author

David James Sycamore, Mar 03 2018

Keywords

Comments

Prime(r) has a056240-type k if A295185(prime(r))=prime(r-k)*A056240(prime(r)-prime(r-k)).
This sequence lists primes having a056240-type k=2, each having form ~2(g1,g2) where g1 is the first gap below prime(r), and g2 is the second (notation explained in A295185). The majority of primes appear to be of a056240-type 1.

Examples

			211 is included because the smallest composite number whose sum of prime factors (with repetition)=211 is 6501=197*33, a multiple of the second prime below 211, not the first. 211~2(12,2) is the smallest prime to have this property. Likewise 541~2(18,2), 1693~2(24,2), 2879~2(18,4), etc.

Crossrefs

Cf. A056240, A295185, A299110, A293652, A299912, A300359.

Extensions

Edited by N. J. A. Sloane, Mar 10 2018

A300359 Primes of a056240-type 3.

Original entry on oeis.org

4327, 6947, 7283, 7993, 11863, 12143, 14591, 15859, 18583, 18617, 18839, 18947, 19661, 20593, 21059, 21313, 22853, 29567, 29917, 31307, 32531, 32831, 35023, 35491, 35671, 39499, 39659, 40343, 41579, 42821, 43853, 46807, 48221, 49253, 49363, 49597, 50411, 51109, 52667, 54401
Offset: 1

Views

Author

David James Sycamore, Mar 03 2018

Keywords

Examples

			4327 is included because the smallest composite number whose sum of prime factors (with repetition)=4327 is 526809=4283*123, a multiple of the third prime below 4327 (not the first or second). 4327~3(30,8,6) is the smallest prime to have this property. Likewise 6947~3(30,6,4), and 7993~3(30,12,2).

Crossrefs

Cf. A056240, A295185, A299704, A293652, A299912, A300334.

Extensions

Edited by N. J. A. Sloane, Mar 10 2018
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