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A102020 Indices of primes in sequence defined by A(0) = 17, A(n) = 10*A(n-1) - 13 for n > 0. 1
0, 1, 4, 6, 18, 19, 27, 57, 249, 396, 7590, 32715, 70977, 83586 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Numbers n such that (140*10^n + 13)/9 is prime.

Numbers n such that digit 1 followed by n >= 0 occurrences of digit 5 followed by digit 7 is prime.

Numbers corresponding to terms <= 396 are certified primes.

a(15) > 10^5. - Robert Price, Jan 17 2015

REFERENCES

Klaus Brockhaus and Walter Oberschelp, Zahlenfolgen mit homogenem Ziffernkern, MNU 59/8 (2006), pp. 462-467.

LINKS

Table of n, a(n) for n=1..14.

Makoto Kamada, Prime numbers of the form 155...557.

Index entries for primes involving repunits.

FORMULA

a(n) = A102937(n+1) - 1.

EXAMPLE

157 is prime, hence 1 is a term.

PROG

(PARI) a=17; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-13)

(PARI) for(n=0, 1500, if(isprime((140*10^n+13)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A102937.

Sequence in context: A113610 A263319 A062046 * A125133 A109310 A219190

Adjacent sequences:  A102017 A102018 A102019 * A102021 A102022 A102023

KEYWORD

nonn,hard,more

AUTHOR

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 28 2004

EXTENSIONS

7590 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008

a(12)-a(14) derived from A102937 by Robert Price, Jan 17 2015

STATUS

approved

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Last modified June 24 10:10 EDT 2019. Contains 324323 sequences. (Running on oeis4.)