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A320708
Indices of primes followed by a gap (distance to next larger prime) of 20.
3
154, 259, 442, 480, 548, 753, 777, 783, 876, 971, 1035, 1066, 1095, 1106, 1147, 1254, 1277, 1302, 1337, 1345, 1355, 1381, 1396, 1400, 1423, 1438, 1562, 1592, 1613, 1662, 1669, 1808, 1955, 2016, 2043, 2081, 2116, 2129, 2147, 2226, 2302, 2307, 2387, 2517, 2547, 2563, 2694, 2724, 2745, 2755, 2766
OFFSET
1,1
COMMENTS
Indices of the primes listed in A031938.
FORMULA
a(n) = A000720(A031938(n)).
A320708 = { i > 0 | prime(i+1) = prime(i) + 20 } = A001223^(-1)({20}).
MATHEMATICA
Select[Range[3000], Prime[#] + 20 == Prime[# + 1] &] (* Vincenzo Librandi, Mar 22 2019 *)
PROG
(PARI) A(N=100, g=20, p=2, i=primepi(p)-1, L=List())={forprime(q=1+p, , i++; if(p+g==p=q, listput(L, i); N--||break)); Vec(L)} \\ returns the list of first N terms of the sequence
(Magma) [n: n in [1..3000] | NthPrime(n+1) - NthPrime(n) eq 20]; // Vincenzo Librandi, Mar 22 2019
CROSSREFS
Equals A000720 o A031938.
Row 10 of A174349.
Subsequence of A107730 (prime(n+1) ends in same digit as prime(n)).
Indices of 20's in A001223.
Cf. A029707, A029709, A320701, A320702, ..., A320720 (analog for gaps 2, 4, 6, 8, ..., 44), A116493 (gap 70), A116496 (gap 100), A116497 (gap 200), A116495 (gap 210).
Sequence in context: A214475 A327775 A053243 * A261377 A256024 A239564
KEYWORD
nonn
AUTHOR
M. F. Hasler, Oct 19 2018
STATUS
approved