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A101720 Indices of primes in sequence defined by A(0) = 43, A(n) = 10*A(n-1) - 7 for n > 0. 1
0, 3, 8, 9, 30, 69, 159, 170, 276, 629, 723, 3716, 5541, 5633, 10655, 12723, 31953 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Numbers n such that (380*10^n + 7)/9 is prime.

Numbers n such that digit 4 followed by n >= 0 occurrences of digit 2 followed by digit 3 is prime.

Numbers corresponding to terms <= 723 are certified primes.

a(18) > 10^5. - Robert Price, May 09 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..17.

Makoto Kamada, Prime numbers of the form 422...223.

Index entries for primes involving repunits.

FORMULA

a(n) = A102985(n+1) - 1.

EXAMPLE

42223 is prime, hence 3 is a term.

MATHEMATICA

Select[Range[0, 300], PrimeQ[(380*10^# + 7)/ 9] &] (* Robert Price, May 09 2015 *)

PROG

(PARI) a=43; for(n=0, 1500, if(isprime(a), print1(n, ", ")); a=10*a-7)

(PARI) for(n=0, 1500, if(isprime((380*10^n+7)/9), print1(n, ", ")))

CROSSREFS

Cf. A000533, A002275, A102985.

Sequence in context: A025615 A297324 A223331 * A093439 A000927 A055513

Adjacent sequences:  A101717 A101718 A101719 * A101721 A101722 A101723

KEYWORD

nonn,hard,more

AUTHOR

Klaus Brockhaus and Walter Oberschelp (oberschelp(AT)informatik.rwth-aachen.de), Dec 14 2004

EXTENSIONS

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

a(15)-a(17) from Kamada data by Ray Chandler, Apr 30 2015

STATUS

approved

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Last modified August 11 15:12 EDT 2020. Contains 336428 sequences. (Running on oeis4.)